B-29 Superfortress and B-17 Flying Fortress together
|
|
The Boeing B-29 Superfortress entered service later than any other genuinely important aircraft of the Second World War. It was also one of the few aircraft to be developed after war broke out in 1939, although Boeing had been working on similar aircraft for most of the 1930s. They had previously been involved in the development of the XB-15, a massive aircraft in its own right, but one that had proved to be too heavy for its own engines.
Work on very large bombers had continued with the Douglas XB-19, an aircraft with a wingspan of 212ft and a loaded weight of 160,000lbs and powered by four 2,000hp Wright Cyclone engines. Work started on this aircraft as the XBLR-2 (Bomber, Long Range) in 1935, but it did not make its first flight until 27 June 1941. Performance was disappointing, with a top speed of 224mph, and when the AAF issued the specifications that would lead to the B-29 they were less ambitious that those for the XB-19, which would remain the largest bomber in the world until the appearance of the Convair B-36.
The official specifications issued in 1940 called for a bomber with a top speed of over 400mph, a range of 5,300 miles and a payload of 2,000lb. This would have given the resulting aircraft an operational radius of around 2,500 miles, in line with an Air Board suggestion of June 1939 for bombers with a radius of operations of 2,000 miles and 3,000 miles. In the event the B-29 would have a lower speed and shorter range but much higher bomb load. By the time the B-29 entered service the need for such a long range was gone – Britain had survived, and so could be used as a base for attacks on Germany, and American troops were advancing across the Pacific, capturing islands that could be used for the attack on Japan.
Four companies produced designs to match the Army requirements. The Douglas XB-31 and Lockheed XB-30 never progressed beyond scale models, but both the Consolidated XB-32 and Boeing XB-29 would enter production.
Boeing had continued to work on large bomber designs after the XB-15 and B-17 were ready. In the late 1930s they had produced the Model 334, a four engined bomber with tricycle landing gear, a pressurised cabin, a twin vertical tail and powered by experimental flat liquid cooled Wright engines. In July 1939 they produced the Model 334A, which featured a circular fuselage, an all glass nose and a single large vertical tail and was powered by four Pratt & Whitney R-2800 radial engines. This model also featured a high aspect ration wing (long and thin).
Boeing’s initial response to the Air Corps specifications was the Model 341, similar to the Model 334A but with a huge 124ft 7in wingspan. During the spring of 1940 the Air Corps modified their requirements to take into account British and French combat experience. The new specifications called for self-sealing fuel tanks and more guns and added 9,000lb of weight to the aircraft. Boeing responded on 11 May 1940 with the Model 345, an even larger aircraft with a wingspan of 141ft 2 in (still down slightly on that of the XB-15). This new model would be the basis of the B-29.
Boeing received an order for two prototypes on 24 August 1940. This was expanded to include a third aircraft in December, then on 17 May 1941, even before the first prototype had taken to the air Boeing received an order for 14 YB-29 service test aircraft and 250 production B-29s. The aircraft was also now officially named the Superfortress, to emphasis its superiority over the B-17 Flying Fortress. The production order was doubled to 500 in January 1942.
The first XB-29 prototype took to the air on 21 September 1942. It was the first fully pressurized bomber, the first bomber with a successful system of remote controlled turrets linked to a central fire control system and the heaviest aircraft yet to enter production. The unarmed XB-29 had a top speed of 368mph, a range of ver 5,800 miles and a ceiling of over 32,000 feet.
A number of different combinations of guns were tried out before the eventual system was adopted, with four remote controlled turrets on the main fuselage (two above and two below) and a manned turret in the tail. Each turret carried two .50in machine guns, while the tail turret also carried a 20mm cannon. On the XB-29 and YB-29 aircraft these guns were aimed from a number of teardrop shaped sighting blisters using periscopic gun sights.
The XB-29 project suffered from serious problems with the Wright R-3350 engines, which tended to overheat, causing fires. On 18 February the second prototype was destroyed after one of those fires burnt through the main wing spar. Eddie Allen, the Boeing test pilot, his crew and a number of fire fighters were killed in the crash. This delayed the program somewhat and forced the Army Air Corps to take direct control. Work continued, and the first YB-29 was ready on 15 April 1943, and the first production aircraft followed soon afterwards.
