XB-29 Model 345
Model 345 was a mid-wing monoplane with a pressure cabin and was designed for four Wright R-3350 engines. It retained most of the design features of Model 341, including the new wing, with its "Boeing 117" aerofoil. It had a double-wheeled tricycle undercarriage, but the main wheels retracted into the inboard nacelles instead of into the wing, as in previous models. The gross weight went up to 112,300 lb. and the bomb-load to eight tons. But the biggest advance of all was made in defensive armament. Model 341 had had only six 0.50m. manually operated guns ; Model 345 had ten 0.50m. guns, and one 20 mm. cannon mounted in the tail. The machine guns were mounted in pairs in four Sperry retractable turrets and one power-operated tail turret. The problem of pressurising the gun positions was tackled in an entirely new way, and instead of the gunners sitting inside each turret, they controlled the guns remotely from easily pressurised compartments inside the fuselage, sighting through periscopes.
Towards the end of 1940 a full-size wooden mock-up of the XB-29. was built and inspected by USAAF technicians. At the same time wind-tunnel testing of detailed scaled-down sections was started. Meanwhile the Wright engine had been accepted for production by the American Material Command, and so Boeings were able to start detail design work on the nacelles and mountings for this engine. This was not as easy as it sounds. The aerodynamics people had conceived a beautifully clean nacelle, offering a minimum of head resistance ; the project engineers realised that the nacelles would have to house two turbo-superchargers for each engine, inter-coolers and the main undercarriage legs and wheels, as well as the engines themselves. Clay models were made and tested for more than a year before a compromise was finally arrived at, and as a result it is claimed that more air is drawn through B-29 nacelles than any others of their size.
Every other component of the XB-29 was designed with equal care. To reduce pilot fatigue on long flights, great attention was paid to ease of control, and all movable surfaces were tested thoroughly in the wind tunnel before being accepted. New aerofoil sections were evolved and special automatic control tabs built, which ensured that operation of the control surfaces required no more effort than those on much smaller aircraft. Pilots report very favorably on the B-29. The large fin ensures directional and lateral stability even under the extremely adverse condition of two engine failures on the same side at take-off.
The bomb-bay of the XB-29 was originally designed to accommodate a number of large bombs. By redesigning it, Boeing engineers made it capable of accommodating an equal weight of smaller, more bulky bombs, and at the same time increased the bomb-load to ten tons. The armament was changed from the Sperry retractable to the Sperry non-retractable type of turret, and the inboard portion of the wing trailing edge was redesigned to improve the flap characteristics. Apart from these features, however, the prototype XB-29 was almost identical with the original Model 345 design.
On September 21st, 1942, the prototype took off for the first time, in the hands of Eddie Allen, Boeing's chief test pilot (since dead). It bore out its designers' hopes in every respect and was put into production in the greatest manufacturing programme ever devoted to a single aircraft type. Four major companies — Boeing, Martin, Bell and Fisher — operating six of the largest aircraft plants in America, were ordered to build the airframes. The engines were to be built by Wrights and Dodge, the latter company operating the largest single-unit manufacturing plant in America.
The principal changes made for production were the use of General Electric turrets, sighted from "astro-domes," instead of the Sperry periscopically sighted type, and the fitting of four-bladed airscrews. The loaded weight has increased a further 20,000 lb. to 135,0001b., but the production machines could still fly at over 350 mph and had a range of over 4,000 miles with some bombs.
The perfection of the design was achieved by its high cost: the B-29 cost 894 thousand dollars - almost 4 times more expensive than the B-17.
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