Boeing XB-39 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.

XB-39 Superfortress
Boeing XB-39 serial number: 41-36954
General information
Type	Heavy bomber
Manufacturer	Boeing
Primary user	United States Army Air Forces
Number built	1
History
Manufactured	1944
First flight	9 December 1944
Developed from	Boeing B-29 Superfortress

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

General Motors modified B-29 to use Allison V-3420 engines

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: 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: 1,736 sq ft (161.3 m²)
• Empty weight: 74,500 lb (33,800 kg)
• Gross weight: 120,000 lb (54,000 kg)
• Max takeoff weight: 133,500 lb (60,560 kg)
• Powerplant: 4 × Allison V-3420-11 liquid-cooled W24 (double-vee) engines, 2,100 hp (1,600 kW) each

Performance

• Maximum speed: 405 mph (648 km/h, 351 kn)
• Range: 6,290 mi (10,060 km, 5,460 nmi)
• Service ceiling: 35,000 ft (11,000 m)
• Wing loading: 69.12 lb/sq ft (337.5 kg/m²)
• Power/mass: 0.073 hp/lb (121 W/kg)

Armament

• Guns:
  
  • 8× .50 in (12.7 mm) Browning AN/M2 machine guns in remote controlled turrets
  • 2× .50 in (12.7 mm) AN/M2 machine guns in manually-operated tail turret
  • 1× 20 mm (0.787 in) AN/M2 cannon in tail turret
• Bombs: 20,000 lb (9,000 kg)

See also

Related development

• Boeing B-29 Superfortress
• Boeing B-50 Superfortress

Related lists

• List of bomber aircraft
• List of military aircraft of the United States

References

Bibliography

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

External links

• XB-39 on USAF Museum site