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Boeing B-17 Flying Fortress - Design

The Boeing B-17 Flying Fortress was the anvil against which the USAAF fighter force hammered the Luftwaffe fighter arm to destruction in the skies over Germany. That was of immense strategic importance above and beyond the destruction that B-17s visited on military and industrial targets, threatening a level of damage to key industries that the Third Reich's leaders could not tolerate. The B-17 was a singular design for which there would have been no viable substitute until the B-29 became available in quantity. if Boeing could have designed the B-29 without the experience gained from the B-17. Even before long range fighters were available to escort deep penetrations, massed formations of B-17s took a significant toll of the Luftwaffe fighter arm, both physically and psychologically, helping to make subsequent the German air arm's recovery impossible.

The B-17s range was particularly sensitive to its bomb load. The B-17B boasted a maximum bomb load of 8,800 lbs and a range of 3,000 miles, but it could not accomplish either in combat conditions, in which it nominally carry only 2,400lbs up to 1,500 miles.

The B-17 was an uncompromising 1934 design intended to produce the fastest, highest-flying heavy bombardment aircraft extant. Boeing's design team adopted those objectives in response to stated Army Air Corps requirements, but pushed them to the limit as a conscious high risk, high gain strategy to deliver blows against an industrial enemy. For a variety of reasons involving internal Army politics and blind luck-the loss of the first prototype to a pilot error accident-that strategy nearly failed and most of the initial Army bomber contract went to the mediocre twin-engine B-18, a derivative of the DC 3 civilian transport. That having been said, Boeing's boldness reaped huge strategic dividends in range, bomb load and ability to absorb battle damage.

It had many problems, since no such bomber had been produced in the United States. Four enginebombers such as the Russian Sikorsky had been produced in small quantities; however, these had also been slow, clumsy and relatively underpowered. Numerous improvements in the design were included in the XB-17. To reduce air resistance, bombs were carried internally. Other improvements included housing the pilot and crew members inside the fuselage in heated sound-proofed quarters; installing additional machine guns to fire from enclosures in the fuselage; adding new Hamilton constant-speed propellers which would improve the versatility of the operation; developing tabs for the rudder and elevators to assure easier trim control; and eliminating all protruding surfaces as far as possible in order to provide an aerodynamical clean aircraft.

The excellent Wright R-1820 nine-cylinder engine, re-engineered at Air Corps insistence to burn 100-octane gasoline, was an essential cornerstone of the B-17's success. Another was the development of the turbo supercharger by General Electric on an Air Corps contract, the only discrete Army research and development program to receive funding through the Great Depression. The importance of the turbo supercharger lies in the fact that the War and Navy departments stopped subsidizing the development of military aero engines during the Great Depression. American military aircraft would henceforth be powered by engines designed for civilian use, and while high altitude performance had obvious military importance it had little civilian value.

The European solution, gear-driven superchargers designed as an integral part of the engine, was an obvious non-starter for economic reasons. The military market was simply too small. American superchargers therefore would be add-on accessories and the only evident way to power such a supercharger was a turbine driven by engine exhaust gasses. The extremely high temperatures and rotational speeds to which the turbines were subjected posed obvious problems. A further complicating factor was the lack of full-sized high altitude wind tunnels: turbo superchargers could only be tested in actual flight with the obvious risks that entailed. Beginning work in 1919, General Electric eventually surmounted these problems and by the mid-1930s was fielding increasingly reliable turbo superchargers. The B-17 was slated for them from the beginning.

The result was a bomber capable of delivering a two-ton bomb load over a thousand miles from its base-the figures are approximate, based on data from missions flown over Germany in 1943-44-penetrating enemy air defenses in formation at altitudes of 25,000 to 29,000 feet. The emphasis in the preceding sentence is warranted since a formation's speed and ceiling are dictated by its most poorly performing aircraft. Such performance, unprecedented in the mid- to late 1930s, speaks volumes both for the soundness of the B-17's design and for the excellence of Wright, General Electric, and Boeing production line quality control. The excellence of the B-17's design is highlighted by comparing it to that of the Consolidated B-24 Liberator, the closest thing to an available substitute. A newer design by five years and similarly powered, the B-24 was nonetheless inferior to the B-17 in every critical performance parameter that counted in the European theater of operations save maximum range.

A final factor contributing to the B-17's success was the decision by the Air Corps during the 1920s to adopt the .50 caliber machine gun as its standard aircraft weapon. Designed toward the end of teh Great War as a heavy infantry machine gun, the Browning .50 caliber was an uncompromising design with exceptional ballistic performance. Not only was its projectile nearly four times as massive as that of .30 caliber weapons, its superior ballistic coefficient and streamlined shape gave it the best velocity over distance characteristics of any commonly used aerial machine gun of World War II. As a result, the Fortress's effective defensive fire ranged well beyond the practical hitting distance of any Axis air-to-air gun. While unescorted B-17 formations proved unable to sustain deep penetrations of German airspace without incurring prohibitively heavy losses, they inflicted serious losses on the German fighter arm in the process. To be sure, the Air Corps initially underestimated the need for defensive armament and Boeing engineers resisted the addition of turrets that spoiled the aircraft's aerodynamic shape. Ultimately, however, tactical logic and superior engineering prevailed and from early 1942 on B-17s were well provided with heavy defensive armament, much of it mounted in power-operated turrets.

Each four-engine Flying Fortress had a crew of 10: pilot, co-pilot, navigator, bombardier, radio operator, and five machine gunners spread around the aircraft. To defend themselves from fighter attacks, the crew manned 13 .50-caliber machine guns. The crew size later dropped to nine when Army Air Forces officials decided that one waist gunner could operate machine guns on each side of the aircraft behind the wings. One gunner, usually a crew's smallest enlisted man, sat in the ball turret, a rotating sphere on the aircraft's belly equipped with two machine guns. It offered the aircraft's best view of fighters approaching from below.

Many unanticipated problems had to be overcome. Minus 30 to 50 degree temperatureswere quite normal over northern Europe at 25,000 feet, the optimum altitude for heavybombers. Frostbite, the predominant hazard, made the provision of suitable flight crew clothing essential. The USAAF had developed electrically heated flying suits in 1940, but they only became available to gunners during the winter of 1942-43. The suits were completely unreliable, when subjected to the rigors of operational flying.

On one occasion, a lightly loaded Fortress was flown to 36,000 feet where the lack of atmospheric pressure brought additional discomforts for crew members bladder incontinence. With this new problem came an innovative fix designed by the RAF. To avoid wet clothing which consequently froze, a rubber tube was rigged up to run down inside the leg of the flying suit and then out of the flying boot onto the cockpit floor.

Facilitated by intercom and radio connections, the Fortress' aircrew arrangement throughout the cockpit, crew compartment and fuselage ensured that the dispersed crewmen retained their group cohesion in air to air combat. Compartmentalized responsibilities and specialized training demanded aircrew discipline in coordinating defensive fire and fighting battle damage to the airframe, engines and subsystems. This dispersed crew arrangement provided for more defensive armament that could protect the bomber from all flight attitudes of fighter attack, especially with the B-17G modified "chin" turret model giving frontal attack defense.

The size of the airframe and engineering capacity enabled the Fortress to grow in defensive firepower from ten .50 caliber machine guns aboard the "E" model to the "G" model with thirteen machine guns providing all around defense. The porcupine firepower gave large formations of B-17s overlapping fields of fire that enabled adjacent elements and squadrons to cover one another. The result was the fifty-four aircraft combat box formation that dealt severe blows against the industrial strength Nazi Germany.



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Page last modified: 07-09-2018 07:20:50 ZULU