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.
Design and development.
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.
Operational history.
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.
Specifications (XB-39).
General characteristics* Crew: 10: * Length: ()* Wingspan: ()* Height: ()* Wing area: ()* Empty weight: ()* Gross weight: ()* Max takeoff weight: ()* Powerplant: 4 × Allison V-3420-11
 liquid-cooled W24 (double-vee) engines, 2,100 hp (1,600 kW) 
Performance* Maximum speed: (, ) * Range: (, ) * Service ceiling: ()* Wing loading: ()* Power/mass: 0.073 hp/lb (121 W/kg)
Armament
See also.
Related development
Related lists
References.
Notes.
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