Боинг XB-39: Экспериментальный бомбардировщик с альтернативной силовой установкой

Prototype bomber aircraft by Boeing

The Boeing XB 39 Superfortress was a United States prototype bomber aircraft, a single example of the B 29 Superfortress converted to fly with alternative powerplants. It was intended to demonstrate that the B 29 could still be put into service even if the first choice of engine, the air cooled Wright R 3350 radial engine, ran into development or production difficulties.

Starting life as the first YB 29 delivered to the United States Army Air Forces, it was sent in November 1943 to the Fisher Body Aircraft Development Section of General Motors to be converted to use Allison V 3420 17 liquid cooled W24 (twin V12, common crankcase) inline engines. Fisher was chosen for the modification as it was familiar with the engine, as it was to power the P 75 Eagle that they were then developing. Testing on it began in early 1944. Further development of the engine and the aircraft was delayed by a series of changes in the planned turbosuperchargers, as the originally specified GE Type CM 2 two stage turbosupercharger became unavailable due to demands on GE's production of its other turbosuperchargers. Other turbosuperchargers were considered, but the result was that the first flights of the XB 39 had to be made without any turbosuperchargers at all.

Fisher finally focused again on the B 39. The first flight of the B 39 was made on 9 December 1944 at Cleveland, Ohio. The initial flight tests of the B 39, without turbosuperchargers installed, were impressive. However, the B 39 program was by now seriously delayed, and the flawed R 3350 B 29s had already been rushed into combat in June 1944. Despite continuing problems with the B 29s, the aircraft was functioning well enough in combat that it no longer made any sense to shift resources in the manufacturing base to a new engine for the B 29 and so the B 39 was not ordered into production.