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Weapons of Mass Destruction (WMD)

B-29 Development

As early 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 production. At first this development work merely took the form of designing "scaled-up" versions of the B-17 with increased 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 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 performance, and its top speed of 200 m.p.h., its rate-of-climb and operational ceiling were all inferior to those of the smaller 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 to conceive something entirely 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 defense for the bomber became a prime consideration, 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 defense 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 armored 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 Boeing. 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 requirements, Boeing 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 provision for a tricycle undercarriage, the first Boeing design to incorporate this feature. It was realised that very large main wheels would be needed to support the 89,900 lb. of the project, and it would have been impossible completely to retract them into the engine nacelles. Consequently each main undercarriage leg was designed with two proportionally smaller wheels, which could be accommodated in the nacelles.

Meanwhile the Army Air Forces asked Boeing to submit plans for pressurising the cabin of the B-17 Flying Fortress. This project — Model 322 — used standard B-17 wings, engines and tail unit, but had a large-diameter, circular fuselage to facilitate pressurisation. Once again it was decided to use a tricycle undercarriage, as the center of gravity had moved forward considerably as a result of the new fuselage layout. But this time twin nosewheels were to be used, in conjunction with standard B-17 main undercarriage units. Model 322 was designed for a maximum bomb-load of 9,928 lb., and its four 1,400 h.p. Pratt and Whitney R.2180 engines would have given it a top speed of about 307 mph at 25,000ft. But, because of the difficulty of pressurising gun positions, only four machine guns were provided for defense, and these were hardly sufficient for a heavy day-bomber. This deficiency, combined with the fact that several other difficulties were experienced with the design of the pressure-cabin, finally compelled Boeings to tell the USAAF that the pressurised B-17 wasn't considered a practical proposition.

But, at last, Boeing knew just what sort of heavy bomber the War Department was after, so they set to work with renewed enthusiasm to produce a bomber that would be entirely new and revolutionary, and not just a re-hash of an existing type. They were not at that time concerned with producing a bigger bomber than the B-17, or even in increasing bomb-load. All they wanted was a pressurised bomber with a good performance at high altitude.

For the first time they decided to use liquid-cooled, in-line engines. Allison had just produced their new 1,150 h.p. V-1710 engine—America's first attempt at producing a modern high-powered, in-line engine. Without any air-cooling worries to think about, Boeing went all-out on cleanliness of design and mounted the four Allisons in tandem pairs on the new Model 333. In fact, this project had cleaner lines than anything before conceived and seemed quite promising. A single-wheeled tricycle undercarriage was called for, and for the first time a tail machinegun position was introduced. But perhaps the most significant feature of Model 333 was the fact that the pressurised sections of the fuselage were connected by a crew communication tunnel of small diameter — a feature that remained unchanged through all the succeeding projects, and was standard on the B-29.

The advantages of the tandem engine installation were, however, questionable, and so this design gave way to Model 333A. This was identical in almost every respect with the Model 333 and used the same Allison engines, only this time they were completely submerged in the wings, the propellers being carried forward of the wing leading edges on long shafts. The sumps of the outer engines protruded below the wing skin and were enclosed in streamlined blisters. Unfortunately, it was soon found that the power of the Allison engine fell off very rapidly at height, and it was decided to try a different engine installation.

Both the Wright and Pratt and Whitney engine companies were working on the development of "flat" type engines that could be completely submerged in the wings of large aircraft, and so Boeing designed a project for each engine. Model 333B used four 1,850 h.p. Wright 1800 engines and had an airframe identical with that of the Model 333A. The larger engines put the estimated loaded weight up to 52,180 lb. and gave a top speed of 364 m.p.h. at 20,000ft. Unfortunately, Model 333B had an estimated range of only 2,500 miles with one ton of bombs, as the submerged engines took up space normally used for fuel tanks. As America had always been "Pacific war minded," where distances are great, a range of 2,500 miles was hardly sufficient for a heavy bomber.

The project with Pratt and Whitney 1800 "flat" engines — Model 334 — was basically similar to Model 333B, but the wing was redesigned to increase fuel capacity, and this type had an estimated range of 4,500 miles. By this time, March 1939, European air forces were being re-equipped with new and deadly monoplane fighters, the Hurricane and Spitfire with eight machine-guns, and the Messerschmitt 109 with cannons and machine-guns, being outstanding. Consequently, the question of defensive armament became of prime importance, and Model 334 was designed with twin fins and rudders in an effort to improve the arc of fire of its eight machine-guns. This further enhanced the already-clean lines of the fuselage and, combined with# the improved efficiency of the wing, permitted an increase in bomb-load of one ton (to 7,830 lb.) and put the estimated top speed up to 390 m.p.h. at 20,000ft. It was found, however, that the completely submerged flat engines did not improve aerodynamic efficiency sufficiently to justify plication of wing structure, and so Model 334 was Boeing's last attempt at using submerged engines. The twin tail was also abandoned, as any small gains in arcs of fire were countered by a lessening of structural efficiency.

Boeing engineers were beginning to realise that the generally improved bomber they were after would not be obtained by such devices as creative engine installations, and they set to work developing instead a new aerofoil that would give greatly improved all-round efficiency. The prototype Consolidated XB-24 Liberator, with its high-aspect ratio Davis wing, must have helped to convince them that this was the wisest policy to pursue, and the next project had a wing very reminiscent of that on the XB-24. Also, as in the XB-24, the main wheels of the tricycle undercarriage were now designed to fold sideways into the wings instead of into the nacelles.

The project designed around the new wing was designated Model 334A, but it was such a radical redesign that it should really have borne a new Model series number. It was the first blood ancestor of the Superfortress, with a high single fin and rudder and a long dorsal fin. By this time the Wright 3350 engine was coming along nicely, and so Boeing decided to use four of these engines on the new project. It was obvious that production engines would develop over 2,000 h.p., and it was estimated that the project would have a speed of 390 m.p.h. at 16,000ft. and a useful 5,333 mile range. The years of painstaking development work were beginning to show results and, although Model 334A was not built, Boeing knew that they were working on the right lines.

Late in 1939 the USAAF asked several aircraft companies to submit designs for a larger long-range bomber, indicating minimum requirements for performance, load carrying capacity, weight, range and armament. This was put out in the form of a formal specification in January 1940. The first few months of the war in Europe had been sufficient to convince the USAAF that their existing bombers and fighters were hopelessly out of date, and consequently Boeing was given plenty of encouragement in developing their heavy bomber designs.

The Aerodynamics Unit was still busy developing improved high-lift aerofoils. It had drawn up the preliminary design of a new 124ft. 7m. span wing of very high aspect ratio, using an aerofoil that gave better results than any previously designed for a bomber. Boeing Model 341, powered by four 2,000 h.p. Pratt and Whitney R-2800 engines, was designed to use this wing and was radically different from anything previously developed, although its fuselage lines bore a "family likeness" to Model 334A. Model 341 had a greatly improved estimated performance over any of the previous designs and, although its loaded weight of 85,672 lb. was about 30 percent, greater than that of Model 334A, it was 15 m.p.h. faster, had a 1,700-mile better range and could carry a 25 per cent, greater bombload.

In the first three months of 1940 Boeing submitted several progressive proposals on this model to the War Department and appeared almost certain of a contract. But once again their luck was out. It was obviously only a matter of time before America would be forced into the war, and so the USAAF increased the size and armament requirements of its specification, and Model 341 was out of the running.

For the next two months the Boeing design staff worked furiously on the new specification and, on May 11th , submitted to Wright Field the preliminary designs of Model 345 — an enlarged version of Model 341, with revised armament. This was quickly accepted by USAAF representatives as being the type desired, and on June 14th, the day on which Paris fell to the Germans, Boeing was authorised to prepare additional aerodynamic data on the machine. Shortly afterwards a contract was awarded, covering the construction of several prototypes, to be designated XB-29.

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Page last modified: 26-08-2018 04:43:57 ZULU