The Boeing XB-39 was a 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 Wright R-3350, ran into development or production difficulties.

Starting life as the first YB-29 delivered to the USAAF, it was modified by General Motors to use Allison V-3420-17 liquid-cooled inline engines. Testing on it began in early 1944. Since no problems with the R-3350 developed, and most flaws of the B-29 were worked out by the time production began, the XB-39 was never ordered.

Specifications (XB-39)

General Characteristics

• Crew: 10: Pilot, Co-Pilot, Flight Engineer, Bombardier, Navigator, Radio Operator, Side Gunners (2), 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: 74,500 lb (33,793 kg)
• Loaded: 120,000 lb (54,432 kg)
• Maximum takeoff: 133,500 lb (60,560 kg)
• Powerplant: 4× Allison V-3420-11 Vee engines, 2,100 hp (1,567 kW) each

Performance

• Maximum speed: 405 mph (648 km/h)
• Range: 6,290 miles (10,064 km)
• Service ceiling: 35,000 ft (10,671 m)
• Rate of climb: ft/min ( m/min)
• Wing loading: 69.12 lb/ft² (337.46 kg/m²)
• Power/Mass: 0.073 hp/lb (0.121 kW/kg)

Armament

• 8× .50 in M2 machine guns in remote controlled turrets; 2× .50 in machine guns in manual turrets; 1× 20 mm M2 cannon in tail
• 20,000 lb (9,072 kg) of bombs

References and Links

• XB-39 on USAF Museum site (http://www.wpafb.af.mil/museum/research/bombers/b3-88.htm)

Related content:

Related development: B-29 - B-44 - B-50

Comparable aircraft: XB-38 Flying Fortress

Designation sequence: B-36 - B-37 - XB-38 - XB-39 - YB-40 - XB-41 - XB-42

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