Martin's airplane can really be considered the world's first serial production high-speed bomber created in the interwar period. The technical breakthrough of the all metal mono–plane reduced many of the structural and performance limitations of bi–planes. Now, aircraft could achieve greater speeds due to lower drag and larger, more powerful engines. Also, larger aircraft could be built that could carry heavier payloads over greater ranges. These advantages of the metal mono–plane were realized in the Martin B–10 bomber, first built in 1932. This two–engine bomber could fly at 207 miles per hour, faster than the best pursuit aircraft at the time.
The experience of the creation of the Boeing B-9 bomber showed that military aircraft will be able to restore lost superiority in speed over passenger ones only if serious measures were taken to improve them. Following the example of passenger aircraft since 1933 on bombers, the cabin of the pilots was closed with a transparent canopy.
The situation with mobile machine guns was more complicated. Sometimes they tried to place a machine gun and an arrow completely inside the fuselage. In order for the barrel to be rotated, slots were made in the body. However, this method did not take root, since the angles of fire greatly decreased, the search for the target and aiming became more difficult. The idea of the machine-gun turrets being advanced from the fuselage during the battle, proposed by G. Junkers at the beginning of the 1920s, did not spread. The most simple and effective way to reduce the aerodynamic drag of small arms of heavy military aircraft was the use of fairings. At first they had the form of screens, then they began to use rotary towers, in which the shooter and the machine gun were covered with a transparent cap, but the stream protruded just the end of the barrel. Turning the turret and moving the barrel of the weapon in a vertical slit-slot, the shooter could lead a defensive fire in almost any direction.
Increased requirements for speed forced designerto abandon the external suspension of bombs. The entire bomb load was to be placed inside the fuselage. In order for the plane to take many bombs, the dimensions of the bomb bay were large. When the bomb bay doors opened, the fuselage appeared to be cut from below on a large length. To preserve the strength of the structure for bending and torsion, the cut-out for bombarding had to be reinforced with powerful power elements. The measures taken to reduce aerodynamic drag made it possible to improve the high-speed qualities of bombers.
The first aircraft with closed compartments of pilots and guns and the internal placement of bombs was the twin-engine Martin B-10. In 1932, when strategic bombing theory was in its genesis, the first all-metal monoplane fighters and bombers came on the line. The B-10 had no external wing bracing, an enclosed cockpit, and retractable landing gear. Its wing was stressed to somewhere around 3 Gs. That bomber’s contemporary fighter, the P-26 (fig. 3), had external wire bracing for its wings, an open cockpit, and fixed landing gear. Its wings were stressed for something like 6 Gs. To anticipate that the United States could develop a cantilever wing that strong, yet thick enough to accept retractable landing gear and machine guns in three years or so would have been extraordinary. As it was, the P-26 could hardly fly faster than the B-10, and the aircraft took a long time to get to bomber altitude because of its slow rate of climb.
By the mid-1930s, the main types of bombers in the US were the B-10 and B-12. Both are different modifications of the same base aircraft "Martin 139". The Martin B-10 was the Air Corps’s first all-metal monoplane bomber and also had an enclosed cockpit, retractable landing gear, and an internal bomb bay. When it entered the inventory in 1934, it was faster than most pursuit planes and could carry a ton of bombs over 1,200 miles.
In the early 1930s, when the biplane planes were everywhere, Martin put forward a project for an advanced machine. According to the scheme, the new aircraft was an all-metal mid-wing. Fashionable at the time, corrugated skin, supposedly improving stability in flight, but eating a portion of the speed, was left only on the top of the fuselage. The crew placement was peculiar. Since the Martin 123 was originally designed for the front screened turret installation, the fuselage was made quite high, providing the pilot, navigator and gunner separate cabs. So the bomber found a unique and well-remembered appearance. As a power unit, Wright SR-1820E (600 hp) motors were chosen.
The first flight of an unarmed aircraft prototype was made in January 1932. When the B-10 prototype XB-907, with an open cockpit, made its maiden flight on February 26, 1932, the results were so poor that the project was nearly canceled. In the first flights, it showed excellent speed characteristics, which the US Air Corps found very interesting. Recommendations from Wright Field and the USAAC included moving the engine nacelle forward to eliminate interference encountered at higher angles of attack; enlarging the wing fillets and turning up the trailing edge of the wing where it blends into the fuselage; reduce the size of the rudder's trailing edge; incorporating a single strut landing gear; sweeping wings back slightly to increase stability and lengthening the wings to 70 feet, 7 inches from the prototype's 62 feet, 2 inches.
USAAC required additional tests at the Wright Field airfield and under the new designation XB-907 prototype was handed over to the military. In addition to minor improvements, the plane received a front turret and an upgraded R-1820-19 engine. After improvements, the takeoff weight increased by 900 kg, but the speed also increased to 322 km/h. On March 20, 1932, the first official flight of HB-907 took place, after which, in October, an additional prototype, designated HB-907A, was built.
The XB-10 was initially the Martin Model 123 (or XB-907) and featured three open cockpits, retractable landing gear and Townend ring engine cowlings. The Model 123 had a top speed of 197 mph -- faster than pursuit aircraft in service at the time! Martin managed a number of firsts. The B-10 was the first all-metal monoplane bomber with retractable landing gear. It was also the first production warplane to be fitted with radio equipment and for all these firsts, Martin was awarded the Collier trophy in 1932 and 48 B-10s were ordered $50,840.00 apiece.
The contract saved Martin from bankruptcy for the time being. Ironically, the B-10, despite its revolutionary firsts, never reached high production numbers. In all, 155 would be procured by the USAAC, divided among B-10s, B-10Bs, and B-12s (B-10s with Pratt & Whitney engines in lieu of the Wright Cyclones). This figure was actually impressive during the Depression, but only kept the production line open for three years. The B-10 series would serve 28 squadrons in the course of a seven year operational career.
The aircraft was returned to Martin in mid-1932 for several modifications designed to make the basic aircraft even better. First, the front gunner's compartment was covered with an enclosed full-rotating dome turret, the first on a U.S. bomber. Next, original engines were replaced by more powerful versions, and the ring cowls were replaced with full engine cowlings giving the aircraft improved streamlining. Finally, the wing span was increased by more than eight feet. The new aircraft was redesignated XB-907A and then XB-10 by the U.S. Army Air Corps and given the serial number 33-139.
The basic design of the Martin B-10 began a revolution in U.S. bomber design and made all existing bombers in Army inventory completely obsolete. The all-metal monoplane with enclosed cockpits and gun turrets, retractable landing gear and speed-enhancing features like full engine cowlings would set a design standard for decades to come. The Glenn L. Martin Co. was awarded the Collier Trophy in 1932 for the XB-10. In fact, the B-10 was so fast it would take pursuit plane designers another three years to produce an aircraft significantly faster than the production B-10.
Fighter planes had dominated the skies and military thinking during the Great War, but that changed quickly when the war ended. In 1921, Billy Mitchell showed that airplanes could sink captured German battleships and popularity shifted from the fighter boys to the lumbering bombers, even then growing bigger and faster. Bomber advocates believed that the more powerful bombers would always get through and that the fighter planes sent against them would be ineffective.
Advances in technology gave weight to their arguments. When the B-10 bomber appeared, it was heavily armed and capable of flying at 235 mph, faster than the P-26 “Peashooter,” the standard fighter of the US Army Air Corps. Major air maneuvers during the early 1930s seemed to prove that “due to increased speeds and limitless space it is impossible for fighters to intercept bombers".
The B-10 (and its variants) formed the bulk of the U.S. Army Air Corps first-line bomber strength throughout the middle 1930s, although many older biplane bombers remained in service. The Air Corps would begin looking for an improved bomber almost as soon as the B-10s entered service. The Army leadership wanted a longer range bomber and would order development of several types beginning in 1935.
But the Air Corps was already looking ahead and, in August 1934, circulated a proposal for a new aircraft. Builders were instructed to have designs ready for a flying competition in October 1935; the desired aircraft would be capable of flying 2,000 miles with a 2,000-lb. bomb load at a speed of 200 mph.
Martin built a good twin-engine bomber, the B-10. Boeing found new business by crafting a much better bomber: the B-17. As revolutionary as the B-10 was, it was soon to be surpassed by even better aircraft like the Boeing Model 299, which was tested in 1935. The Model 299 would lead to one of the most famous bombers of all time -- the B-17. Although the B–10 still lacked the range and payload required for decisive strategic bombardment, Air Corps bomber advocates were encouraged, and they awaited further bomber improvements. The answer for bomber advocates came in 1935 with the four–engine, Boeing B–17 Flying Fortress. Its payload, range and speed provided the AirCorps with the hardware to implement the emerging doctrine of strategic bombardment. General Arnold wrote about the B–17 in his memoirs saying, “This was the first real American air power.” He also described the B–17 as being “for the first time in history airpower that you could put your hand on.”
|Join the GlobalSecurity.org mailing list|