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Boeing XB-39 (Spirit of Lincoln) - United States, 1944

Detailing the development and operational history of the Boeing XB-39 (Spirit of Lincoln) Heavy Strategic Bomber Prototype Aircraft.

 Entry last updated on 9/19/2016; Authored by Staff Writer; Content ¬©

  Boeing XB-39 (Spirit of Lincoln)  
Picture of Boeing XB-39 (Spirit of Lincoln)
Picture of Boeing XB-39 (Spirit of Lincoln)

Intended to test the feasibility of an alternative powerplant, the Allison-powered Boeing XB-39 Superfortress ended its days as a single prototype.

Such was the importance of the Boeing B-29 Superfortress to the United States Army Air Forces during the latter stages of World War 2 (1939-1945) that the program was given several fail safes to keep it a viable heavy bomber product moving forwards. This led to a YB-29 developmental machine being set aside by the USAAF for alternative powerplant implementation - assuming that the required stock of problematic Wright R-3350 radial engines would not be available for one reason or another. A modification process was undertaken by Fisher Body (General Motors) in 1944 on the YB-29 aircraft and this went on to produce the XB-39 "Spirit of Lincoln" bomber prototype.

Its basic form and function remained faithful to the original Boeing design but the powerplants in play were now focused on 4 x Allison V-3420-17 series liquid-cooled engine (the original B-29 relied on the aforementioned Wright air-cooled units). Fisher was also using these engines in its (ultimately failed) P-75 "Eagle" long-range escort fighter. Delays in the intended turbosuperchargers dogged the XB-39 project so the first-flight on December 9th, 1944 was had without these installed - though the aircraft provided a successful demonstration nonetheless.

Despite the promising nature of the large aircraft, the Wright air-cooled radials, warts and all, remained the primary focus of the B-29 production campaign, leaving the XB-39 without a battlefield role or notable buyer. Additionally, Fisher was pushed to commit more and more of its resources to the XP-75 fighter prototype which held higher priority for the USAAF at this point in the war. Decisions led to the ultimately abandonment of the XB-39 project with the single prototype being completed and flown (if only for a short time).

As built, the XB-39 held 4 x Allison V-3420-11 liquid-cooled engines of 2,100 horsepower each able to propel the aircraft to speeds of 405 miles per hour out to ranges reaching 6,300 miles and a service ceiling of 35,000 feet. Its crew numbered ten and the armament suite was similar to that of the original B-29 (including remote-controlled turrets and tail cannon). 20,000lb of drop stores could be carried internally.

Boeing XB-39 Specifications

Service Year: 1944
Type: Heavy Strategic Bomber Prototype Aircraft
National Origin: United States
Manufacturer(s): Boeing Company / Fisher (General Motors)
Total Production: 1

Structural (Crew, Dimensions, Weights)

Operating Crew (Typical): 10
Overall Length: 99.02 feet (30.18 meters)
Overall Width: 141.24 feet (43.05 meters)
Overall Height: 27.72 feet (8.45 meters)

Weight (Empty): 74,516 lb (33,800 kg)
Weight (MTOW): 133,512 lb (60,560 kg)

Power / Performance (Engine Type, Top Speed)

Engine: 4 x Allison V-3420-11 W24 liquid-cooled engines developing 2,100 horsepower each.

Maximum Speed: 351 knots (404 mph; 650 kph)
Maximum Range: 5,432 nautical miles (6,251 miles; 10,060 km)
Service Ceiling: 36,089 feet (11,000 meters; 6.84 miles)
Rate-of-Climb: 1,000 feet-per-minute (305 m/min)

Armament / Mission Payload

10 x 0.50 cal Browning M2 heavy machine guns in four remote-controlled turrets.
2 x 0.50 cal Browning M2 heavy machine guns and 1 x 20mm M2 cannon in tail unit.

Up to 20,000lb of conventional drop stores held internally.

Global Operators (Customers, Users)

United States (cancelled)

Model Variants

XB-39 - Base Project Designation; single, flyable prototype completed.

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