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. ^[1] 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.

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?m ² )
• 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/m ² )
• 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)

Related development

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

Related lists

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

• XB-39 on USAF Museum site