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Aviation History
1946
1946 - 0252.PDF
FLIGHT SUPEKFORT The Qenealogy of a Notable Long-range Heavy Bomber By JOHN W R TAYLOR THE atom-bomb and its implications have formed atopic of world conversation for several months. Butonly brief mention has been made of the aircraft that delivered this terrible new weapon—the Boeing B-29 Superfortress. There is no doubt that the Superfortresses of the American 20th Bomber Command had begun to make the Japanese realise that war is not all fun and glory long before the first atom-bomb was dropped on Hiroshima. For many months hundreds of Superfortresses had flown out almost every day from the Mariannas, each to drop up to eight tons of bombs on the Japanese homeland. It was no mere accident, nor luck, that the U.S.A.A.F. had available, at the right moment, a bomber with a good SUPERFORTRESS EVOLUTION: These seven drawings, and the three on the third page of this article, show the step-by-step development of the Superfortress. All are to the same scale. to conceive something en- tirely new in the way of a heavy bomber. It believed that, to achieve any degree of accuracy, bombing would have to be done in daylight. This, of course, meant that means of defence for the bomber became a prime con- sideration, because it was no use having bombers that could carry the heaviest bomb-load of any aircraft if they were not coming back from their missions. As in Britain at that time, there were two schools of thought on this business of bomber defence. The first believed that the best defence any bomber could have was speed, and they argued that a small, fast bomber—more or less a big bomb with wings and two powerful engines—would have the best chance against enemy fighters and anti-aircraft fire. , The others were in favour of a large four- engined bomber, formidably armed and enough performance, in particular range and load-carrying capacity, to make the long 14-hour round trips. As long ago as 1936 the Boeing staff began to work on the design of a bomber which was eventually to supersede the B-17 Fortress (Boeing Model 299), which had just been put into produc- tion. At first this development work merely took the form of designing "scaled-up" versions of the B-17 Wlth in- creased performance and bomb-load, and little attention was paid to improving operational efficiency by cabin- pressurisation, more effective defensive armament or general cleaning-up of the airframe. The First Stepping-stone The first result of this policy was the 87,600 lb. XB-15, which was built and delivered to the U.S.A.A.F. in 1937. This bomber was very similar in general appearance to the B-17, except for the sharp taper of its 157ft. span wing ; it was fitted with four 1,000 h.p. Pratt and Whitney Twin Wasp engines, as the 2,000 h.p. engines for which it was designed were not available. Unfortunately these were not powerful enough to give the XB-15 a very high per- formance, and its top speed of 200 m.p.h., its rate-of-climb and operational ceiling were all inferior to those of the B-17. Nevertheless, the XB-15 was the first stepping-stone to the B-29, and provided much valuable experience in the design of large bombers It served with distinction as a transport with the American Transport Command during the war, one of its jobs being to fly relief supplies to the victims of the Chilean earthquake. By 1938 the American War Department was beginning MODEL 322 (Span, IO8ft. 7in.) armoured against fighter attack, and with a pressure-cabin to enable it to fly high enough to be out of range of all but the heaviest guns. The American love of big things decided the issue, and, anyway, at that time the Mosquito had not yet proved the soundness of the first line of thought. So, from that moment, work on heavy bomber design was speeded-up in several American plants, particularly at Boeings. Meanwhile the engine manufacturers were keeping pace with the airframe companies, and the prototype, 2,200 h.p. 18-cylinder Wright Cyclone had been built. It was given its first test at Wright Field on June 29th, 1938, and the results proved so promising that development work on the engine proceeded simultaneously with airframe design development. In an effort to satisfy the new War Department require- ments Boeings worked out the design of a high-wing pressure-cabin version of the XB-15, known as Model 316. This project differed considerably from the original XB-15, and it was intended to fit four of the new 2,000 h.p. Wright R-3350 engines. A point of great interest was the pro- vision for a tricycle undercarriage, the first Boeing design
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