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XB-39 Superfortress: Wikis


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XB-39 Superfortress
Boeing XB-39 serial number: 41-36954
Role Test-bed bomber aircraft
Manufacturer Boeing
First flight 1944
Introduced n/a
Status Never mass-produced
Primary user United States Army Air Forces
Produced 1944
Number built 1
Developed from B-29 Superfortress

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 the Fisher Body Aircraft Development Section of General Motors to be converted to use Allison V-3420-17 liquid-cooled X24 (twin-V12, common crankcase) inline engines. 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 end result was that the first flights of the B-39 had to be made without any turbosuperchargers at all.

In addition, in early 1944, due to a sudden realization from the U.S. Army Air Force that it required a long range air superiority fighter, Fisher was directed to focus on its other major project, the P-75 which also used the V-3420. In June, 1944 Fisher received a contract for 2,500 P-75s. However, in October 1944, for a variety of reasons, the P-75 was canceled.

Fisher finally focused again on the B-39. The first flight of the B-39 was made on Dec. 9, 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: pilot, co-pilot, flight engineer, bombardier, navigator, radio operator, side gunners (two), top gunner, and tail gunner
  • Length: 99 ft 0 in (30.18 m)
  • Wingspan: 141 ft 3 in (43.05 m)
  • Height: 27 ft 9 in (8.46 m)
  • Wing area: 1736 ft² (161.3 m²)
  • Empty weight: 74,500 lb (33,800 kg)
  • Loaded weight: 120,000 lb (54,000 kg)
  • Max takeoff weight: 133,500 lb (60,560 kg)
  • Powerplant:Allison V-3420-11 liquid-cooled X24 (double-vee) engines, 2,100 hp (1,600 kW) each



See also

Related development

Related lists





  • Jones, Lloyd S. U.S. Bombers, B-1 1928 to B-1 1980s. Fallbrook, CA: Aero Publishers, 1962, second edition 1974. ISBN 0-8168-9126-5.
  • Whitney, Daniel. Vee's For Victory!. Atglen PA: Schiffer Military History, 1998. ISBN 0-7643-0561-1

External links


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