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Strategic Bombers Before World War II

The planes with which the war was to be fought had yet, with few exceptions, to be developed. Of all the models of aircraft on hand in the Air Corps in September 1939, only one — the B-17 — actually flew as a first-line plane during World War II. The roster of then current aircraft types is completely unfamiliar to Americans who well remember the Mustang (P-51), the Marauder (B-26), the Thunderbolt (P-47), or the Liberator (B-24). In 1939 the B-18 was the standard bombardment plane, the A-17 the standard attack plane, and the P-36 the standard fighter; almost 700 of the 800 first-line combat aircraft of the Air Corps consisted of these three models. By the time of America's entry into the war two years later, all of them would be obsolete.

Throughout the decades of the 1920s and 1930s, aviation technology was beginning to make significant gains. The first breakthrough occurred in engine development. Engineers were able to produce motors of increasingly greater horsepower that were also lighter, smaller, and more reliable.

Streamlining and metal construction were also of crucial importance. Most most aircraft designed before 1930 or so were marked by multiple wings, struts, guy wires, fixed landing gear, uncowled engines, and open cockpits. All of this changed around 1930. Metal became the material of choice in aircraft because of its strength, reliability, and predictability, replacing wood and fabric. Biplanes were out, and new designs were built with a single cantilevered wing—it needed no external bracing or struts. Landing gear was retracted into the fuselage during flight, and cockpits and gun ports were enclosed by Plexiglas canopies. All of these seemingly elementary alterations had a profound impact on drag — thus allowing much greater speed and range while also enabling engines to operate more efficiently.

The B-10 bomber of the early thirties had a service ceiling of 24,400 feet compared to 7,700 feet for the MB-2 bomber of the early twenties. The latter's maximum speed of 98 miles an hour was superseded by the 213 mph speed of the B-10. The normal bomb load had progressed from 1,000 to 2,200 Ibs.

The Martin B-10 was only the beginning of a "long reach" by the U.S. Army Air Corps. Developments subsequently pursued on long-range bombers proved to be extraordinary. Improvements in power plants made possible further developments. Radial engines included the Wright R-1820 of 1934, with 675 hp; the Pratt & Whitney R-2600 of 1938, with 1,500 hp; and the Wright R-3350 of 1941, initially with 2,000 hp, but eventually growing to 2,300 hp. Finally, there was the mighty Pratt & Whitney R4360, producing 3,500 hp; although too late for World War 11, it became an important engine in the postwar era and for a decade thereafter.

Doctrinal developments within the Air Corps which by the 1930's emphasized the need for a long-range bomber. That these developments depended partly upon the assurance that such a plane could be built is hardly less certain than that the new doctrine drew continuing support from the technical progress of commercial aviation and of foreign powers in the development of planes which in themselves forced new considerations.

Building higher performance aircraft required more time and effort dedicated to research and development. One factor affecting the industry on the eve of the war was the lack of production-mindedness in the aircraft industry. Unlike the automotive industry which was used to mass producing, the aircraft industry’s emphasis was on design engineering versusproduction engineering. Production engineering consisted of producing automobiles of a proven design and a given demand, whereas the aircraft industry’s design engineering focused on new concepts and initial production in limited quantities. Although this problem faced the American aircraft industry at the beginning of the war, the demand for military aircraft forced the industry to develop mass production techniques.

For the Army, the heavy bomber represented an opportunity cost worth a much larger force of smaller aircraft. Two-engine attack aircraft (e.g. A-20) or medium bombers (e.g. B-25 or B-26) in larger numbers could prove more useful in direct support to ground campaigns. The B-17 also injected uncomfortable uncertainties into War Department plans. Even if clouds of B-17 formations could destroy enemy industrial targets far disconnected from the battlefield, it was impossible to accurately predict timelines for results. Further, gathering information and planning against such targets wasn’t delineated in the War Department manuals that claimed precedence and watered down the Air Corps’ radical theory of air employment. Every dollar spent on a B-17 rather than a tank or an infantry battalion would be trading away a relative known for an unknown.

New Requirements

1933Project A 5,0002,000200
1933Project B 1,000-2,0002,000200-250
193?Project C ......
1935Project D 5,00016,000250+
1939Circular 39-6401,0002,000 350

After extensive studies made in conjunction with the War Plans Division (WPD) of the General Staff, the Air Corps submitted a program of defense requirements on 15 March 1933. The planners considered such factors as the nation's geographic situation, coast line, and critical areas, the air power of rival states, and the increasing vulnerability resulting from the extended range of aircraft. Here was a genuine attempt to derive aircraft requirements from the tactical and strategic situation of national defense and not, as hitherto, from some such consideration as the number of ground troops available.

The Air Corps proposal visualized aircraft needs as centering upon four major areas — the continental United States and its three vital outposts, Panama, Hawaii, and the Philippines. To each of these areas the planners assigned aircraft in terms of the number of groups or squadrons sufficient to provide the minimum force believed necessary.

The specification that produced the XB-15 began in mid-1933 as Project A. In 1934 the Air Corps wanted a heavy bomber with the speed comparable to the latest foreign pursuit aircraft and with strong fire power to fight off concentrated enemy attacks. The War Department, in the spring of 1934, approved an Air Corps project for developing an experimental bomber.

In April 1934 the USAAC contracted with Boeing and Martin to design a bomber capable of carrying 2,000 lb at 200 mph over a distance of 5,000 miles. Boeing gave the project the name of Model 294, while the USAAC called it the XB-15. Boeing built the plane, the XB-15, which arrived at Wright Field in December 1937 for inspection and testing. The War Department further approved an Air Corps request for a multiengine bomber to follow the B-10. Three companies sent planes to compete at Dayton in August 1935. Martin offered one resembling a big B-10. Douglas sent a two-engine bomber derived from its DC-2. Boeing entered its Model 299, a “Flying Fortress.” Martin's design, the XB-16, was judged inferior by the USAAC before a prototype was built. In mid-1935, the USAAC combined Project A with Project D, a proposal asking for the maximum feasible range into the future. The combined program was designated BLR for Bomber, Long Range, though the next year, the XBLR was dropped.

Surely there was also a Project C, but the details appear to have been lost to the dim mists of antiquity.

There were many flaws in the concept of a design competition. In practice the idea of design competition proved unworkable, for it yielded nothing more tangible than a paper promise to perform. The time allowed for replies to a circular proposal, a few months at best, prevented manufacturers from working up realistic plans to accompany their bids. The winning design generally had to be worked out in detail after the original bids had been returned. Actual costs usually outstripped the estimates initially submitted, since the bidders had little or no detailed data on which to base exact price figures. As a consequence, manufacturers usually lost money on airplanes evolved from design competitions.

Broadly considered, the new procurement policy contrived in 1934 by War Department officials under the goad of congressional criticism had one main characteristic: insistence upon competition. Competition was to apply in the procurement of individual experimental aircraft no less than in the procurement of production quantities. In contracts for aircraft in production quantities the real novelty introduced by the new policy was the requirement that all bids be accompanied by a physical sample of the aircraft to be evaluated. The evaluation of bids for production contracts was to be based on actual performance of the sample airplane as tested in flight.

The Navy sided against the B-17 to protect its own turf. The sea service perceived an existential threat to both its land-based coastal defense and carrier-based aviation missions after the long-legged B-17 spotted the ocean liner Rex 600 miles offshore during an exercise in 1938. Despite challenges of weather, over-water navigation, and possible naval anti-aircraft artillery, the heavy bomber could patrol the vast American coast much more quickly than steaming ships — and with ample firepower to sink intruding vessels.

High Flight

In the nineteen-thirties, technical changes of enormous consequence were occurring in American aviation. Since the reorganization of the Air Corps in 1926, the obsolete biplanes of wood and fabric left over from World War I had been discarded. They were replaced by a succession of all-metal monoplanes with more powerful engines, capable of flying longer distances at greater speeds and higher altitudes.

Already before the war it was known that to increase the altitude of the aircraft it is desirable to increase the relative elongation of the wing - the ratio of the square of its span to the area. This gives a significant reduction in the inductive drag of the bearing surface at the price of only a slight increase in the drag of the frontal (at zero lift) and frictional resistance. As the elongation increases, the lift of the wing increases and, consequently, its ratio to the total resistance increases - the aerodynamic quality. The higher it is, the smaller the angles of attack are needed to maintain a given flight path, and the less power is spent to achieve a given speed.

The ever-increasing demands on the airplane during the Great War with regard to climbing speed and ceiling, necessitated the installation of engines of constantly increasing power, while the carrying capacity and speed at serviceable altitude did not proportionately increase. The reason for this was to be found in the fact that the engine power output decreased with the increase of flying altitude. The principal reason for the falling off of the engine power was the decrease in the density of the air as the airplane rose in altitude. It was possible to maintain the engines at a uniform power up to an altitude of almost 4 km. Beyond this point, falling off of the power was avoided by the use of compressors - superchargers - to furnish the engine with air of sea level density, no matter what the surrounding atmosphere may be - an oxygen mask for the engine.

B-17The dramatic development of the air-cooled spark-ignition engine in the 1930s came unexpectedly from fuels research by the petroleum industry on the branched paraffin, octane. This increase in performance spurred super-charger development to keep engines from starving for air at high altitudes. However, the problems of knock and pre-ignition (faulty combustion within the engine's cylinders) remained. With increased supercharging, effective engine cooling was also a problem.

The decade prior to World War II saw engine power double and then triple due mainly to turbo-superchargers advancing substantially. Adding a turbo supercharger to the engine of the B-17B "Flying Fortress" made it a high-speed, high-altitude airplane. This caused considerable excitement at the time because there wasn't a pursuit ship in the air force that could keep up with it. The Army initially prohibited export of super-charged aircraft.

One weakness of the early Merlin engines in Britain was the lack of power generated by its supercharger. The B-17 had a service ceiling of 38,000 feet, while the British Lancaster bomber topped out at 25,000 feet.

But the premier British aviation journal "The Aeroplane" insisted in 1940 that the B-17 was "technically obsolete in the U.S.A. ... in face obsolete in any country in the world ..." A bit later, C.G.Grey, for thirty years editor of England's foremost aviation journal "The Aeroplane", protested that he had "a rooted objection to having obsolete or conceivably dud American stuff planted on this country by American bluff and guff and blah and baloney. And two prize examples of what American propagandists are trying to do is the boosting of the Boeing four-engine machine" and of "America's frightfully secret bombsite." Greay instisted that he had "lots of American friends" but he felt aggrieved by the influx of the accents of "the East Side of New York Yid, and of the tame niggers of Harlem - but little of the real gentlefolk of America, the old Colonial Aristocracy of the Southern States ..."

The B-17 Series A, B, C, and D all predated Pearl Harbor — the planes which carried destruction to Germany were B-17E's, F's, and G's, and chiefly the last two. The B-17B, had a maximum speed of 268 miles an hour and a cruising speed of 230 miles per hour at an altitude of 25,000 feet. Obviously, operation at high altitudes was extremely important if maximum speed was to be obtained, not to mention the additional encouragement to raise the ceiling subsequently provided by enemy fighter planes and anti-aircraft. The installation of superchargers on the B-17B raised the ceiling 10,000 feet over that of the original 1935 model. Further refinements of supercharger and engine (the horsepower reached 1,200 during the course of the war) gave the B-17G in 1945 an operating ceiling of more than 30,000 feet and a top speed of some 300 miles per hour.

The challenges of high altitude flight were multiple. Atmospheric pressure decreases with increasing altitude. The pressure of the atmosphere, or atmospheric pressure, at sea level is 14.7 psi, while at 30,000 feet the pressure falls to 4.0 psi. When an aircraft is flown at high altitude, it burns less fuel for a given airspeed than it does for the same speed at a lower altitude. This is due to decreased drag that results from the reduction in air density. To take advantage of these efficiencies, aircraft are equipped with environmental systems to overcome extreme temperature and pressure levels.

The air contains the typical 21 percent of oxygen, but the rate at which oxygen can be absorbed into the blood depends upon the oxygen pressure. Greater pressure pushes the oxygen from the lung alveoli into the bloodstream. As the pressure is reduced, less oxygen is forced into and absorbed by the blood. Above 7,000 feet, the oxygen pressure becomes increasingly insufficient to saturate the blood. At 15,000 feet mean sea level (MSL), oxygen transfer to the bloodstream drops to 81 percent of saturation. This typically results in sleepiness, headache, blue lips and fingernails, and increased pulse and respiration. Worse yet, vision and judgment become impaired and safe operation of an aircraft becomes compromised. Remaining at 25,000 feet MSL for 5 minutes, where oxygen transfer to the blood is reduced to approximately 50 percent saturation, causes unconsciousness.

In the troposphere in temperature decreases as altitude increases. The rate of change is somewhat constant at about –2 °C or –3.5 °F for every 1,000 feet of increase in altitude. The upper boundary of the troposphere is the tropopause. It is characterized as a zone of relatively constant temperature of –57 °C or –69 °F. Above the tropopause lies the stratosphere. Temperature increases with altitude in the stratosphere to near 0°C.

A cabin pressurization system must be capable of maintaining a cabin pressure altitude of approximately 8,000 feet or lower regardless of the cruising altitude of the aircraft. This is to ensure that passengers and crew have enough oxygen present at sufficient pressure to facilitate full blood saturation. A key factor in pressurization is the ability of the fuselage to withstand the forces associated with the increase in pressure inside the structure versus the ambient pressure outside. In addition to being strong enough to withstand the pressure differential between the air inside and the air outside the cabin, metal fatigue from repeated pressurization and depressurization weakens the airframe.

The early fighter and bomber pilots flew in an unpressurized cockpit, with only a plexiglas canopy between himself and the long delerious burning blue. The open cockpit was covered by a canopy, and then, in larger aircraft, became an enclosed cabin. The B-17 was an operational airplane, built to fly at an altitude where it would be less vulnerable either to ground fire or to fighters diving on it from above. Without bombs, the B-17 could reach a height of about 30,000 feet. To protect the crews of future bombers from the unpleasant effects of high altitudes, without requiring them to wear bulky garments and equipment which would hamper their mobility, designers planned to enclose them in pressurized cabins.

Precision Targeting

Heavily armed, formations of high speed bombers were considered virtually invincible. The development of the daylight strategic bombing doctrine, while vitally important to American success in World War II, failed to place air superiority as a tenet of air doctrine. The generally accepted idea was that invincible, self–protecting bomber formations would fight through all defenses and destroy targets in the enemy’s heartland. In the early stages of World War II, this idea proved to be disastrous.

American Army Air Corps theorists rejected city-busting, and focused on attacking the enemy’s industrial infrastructure. The modern state was dependent on mass production of military goods such as ships, aircraft, trucks, artillery, ammunition, and uniforms. Moreover, essentials such as electrical power, steel, chemicals, and oil were also military targets and of greatest importance because they were the essential building blocks for other types of manufactured military goods needed to sustain a war effort.

As early as 1919 an Army board identified the most important problem confronting bombing operations as the design of an accurate bombsight. The technological answer was the Norden bombsight. Though never as accurate as the propaganda pretended — it could not “hit a pickle barrel from 20,000 feet” — it was nonetheless the best bombsight available at the time.

Military intelligence organizations had existed for centuries, but the information required was fundamentally different from that needed to plan an air campaign. Spies and intelligence personnel were accustomed to determining the capabilities of enemy weaponry, as well as the location and numbers. All of this information was still necessary as the world stumbled toward war in the late 1930s, but the new air weapon required more. If the doctrine of the Air Corps was to break an enemy’s economy and make it unable to continue to fight, then detailed information was needed on that economy. This was virgin territory for intelligence officers.

Two problems — besides the doctrinal and technological — had to be met and overcome. Airmen had to be able to drop their bombs with great precision, and they needed to drop them on the appropriate targets. These tasks required an intelligence-gathering network and an analytical capability that could tell air commanders if their bombing efforts were having the desired effect on the enemy.

At the outset of the war, it was publicly claimed that "American pilots, planes and bombsights have, rom an altitude of almost 20,000 feet, place five out of seven bombs within a twenty-five foot circle on the ground..." In modern terms, this equates roughtly to a circular error probable [CEP] of 10 meters. On the famous bombing raid against the ball-bearing factories at Schweinfurt in October 1943, the 8th Air Force sent more than 250 B-17 bombers to destroy the target. The bombardiers used Norden bombsights, but only one of every 10 of their bombs landed within 500 feet of their target.

Planning for War

A major change in the command structure occurred in 1935 with the creation of the General Headquarters (GHQ Air Force. This organization centralized control over all tactical air units in the continental United States under one commander. A green light was given to the Air Corps for the development of the long sought bomber.

When the B-17 was delivered in 1937, it was enthusiastically received by the GHQ Air Force as "the best bombardment aircraft in existence, particularly for coastal defense." But under existing circumstances it proved far from easy to win recognition of the need for such a plane in coastal defense, and under existing national policy it was difficult to find any other justification for the long-range bomber than its capacity to contribute to the defense of American coasts.

"Airplanes could be built to fly across the Atlantic with a load of bombs and return, and the B-17 right now could make a one way trip and reinforce allies in Europe," wrote the Commanding General of the GHQ Air Force in November 1937. "With landing fields at Wake and Guam," he continued, "it could fly to the Philippines and Asia. However, our National Policy is defensive, and we do not now consider such possibilities." And yet, in the very year that saw the beginning of another major European war, the Air Corps was engaged in an attempt to secure relief from a prohibition which limited the flight of its planes seaward to l00 miles beyond the shore.

One possible way out of the dilemma imposed by restrictive legislation was to extend the life of all aircraft through administrative action. By keeping aircraft in an active status for more than five years before declaring them officially obsolete, it would be possible to increase the number of first-line aircraft on hand without exceeding the limitation on annual replacements. By arbitrarily declaring the life of all tactical aircraft to be seven years and of all training aircraft to be nine years before classification as obsolete, the Air Corps was following a frugal but hazardous policy. The difficulty after 1935 is suggested by comparative figures on the anticipated longevity of military planes as first-line equipment. As of 1 September 1934 the first-line longevity of Air Corps models was six years for pursuit, attack, and bomber aircraft; eight years for observation planes; and ten years for all others. By 1 September 1939 first-line longevity was estimated at four years for pursuit, five years for attack and medium bomber, six years for heavy bomber and observation, eight years for transport, and ten years for all other aircraft.

An Air Corps board was appointed by Arnold in May 1939 and headed by Brig. Gen. Walter G. Kilner. The report submitted by the Kilner Board on 28 June 1939 contained a comprehensive outline of proposed military characteristics for aircraft, weapons, and equipment that could be procured by 1944, and sketched an administrative plan for a major research and development program to be undertaken in the interval.

Addressing itself to the basic problem of formulating desired military characteristics, the board realistically concluded that "efficient airplanes are a compromise between requirements for military use and technical features of design." Current procedures, it was found, restricted the technical staffs of the Air Corps and the aircraft manufacturers in "determining the best compromise of technical features that will result in the best airplane for military use." For the purpose of facilitating practical compromises between aircraft manufacturers and the Air Corps in the development of new equipment, it was proposed that desired military characteristics be stated in terms as general as was possible. The board gave first priority to the development of liquid-cooled engines of various types with a range in horsepower from 1,500 to 2,400 - a program basic to the improved performance of all classifications of planes. It noted the need also for a 3,000-horsepower engine if a truly long-range bomber were to be developed.

The Kilner Board omitted the mention of projects lying on the frontiers of scientific research - such as jet propulsion, guided missiles, and radio aids. The same concentration on objectives that seemed to be more immediately obtainable was reflected in the subsequent decision to drop the project for a bomber with a 3,000-mile radius from the budgetary estimates for the fiscal year 1941 in order to concentrate on the 2,000-mile-radius bomber.

The report of the Army Air Board submitted in June 1939 recommended the establishment and maintenance of nine major air bases, which would ring the continental United States from New England to the Pacific Northwest. These were to be the main operating bases for the approximately fourteen tactical groups that were to be maintained by the GHQ Air Force. This force, which would contain 104 heavy, 84 medium, and 142 light bombardment airplanes and 324 pursuit planes, was now to have a mission of performing frontier defense, reinforcing overseas possessions, and furnishing expeditionary striking forces within the Western Hemisphere as required.

The chief danger, as the planners saw it in 1939 and indeed throughout the prewar period, was that which, if France and Great Britain were defeated, would come from the projection of German and Italian air power by stages across the North Atlantic or from Africa to Brazil in the southern Atlantic. The continental air bases were therefore to be used not only for home defense but also as springboards for the projection of American air power to meet this danger.

The greater range and mobility of the new combat planes made it undesirable to set up any organization that would require the attachment of air units to relatively small territorial commands and restrict their employment to the confines of these commands.

While the Air Corps favored strategic bombing, this preference was not reflected in iron on the ramp. When World War II broke out in Europe in September 1939, the US Army Air Corps had a grand total of 28 strategic bombers: 26 B-17s, one experimental B-15, and one experimental B-19.

The United States then began to rearm, slating airpower for a large buildup. In May of 1940, foreseeing American involvement in the war in Europe, President Roosevelt called for the production of 50,000 planes. In response to this authorization,four months later, the Materiel Division awarded contracts to Boeing for 512 B-17E’s and to Consolidated for 408 B-24’s. This was the opening of the Air Corps heavy bomber production. Over the next two years, the Air Corps — soon to become the Army Air Forces (AAF) in June 1941 — purchased nearly 21,000 aircraft. Of those 20,914 planes, 373 were strategic bombers: 197 B-17s and 176 B-24s.

Paramount among the aviation events of World War II was America's strategic bombing campaign against the Axis nations. For three years, the United States Army Air Force (USAAF), in conjunction with its British allies, conducted an exhaustive and expansive effort to defeat the Axis nations by strategically bombing their war-making and industrial capacities. This air assault forever changed the way the world viewed airpower and created an entirely new dimension in the execution of war. Despite America's self-professed moral proclivity, area and fire bombing civilians eventually became the mode of operation for the American bombing campaign.

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Page last modified: 07-09-2018 07:20:45 ZULU