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Boeing XB-39 (Spirit of Lincoln)

Heavy Strategic Bomber Prototype Aircraft

Intended to test the feasibility of an alternative powerplant, the Allison-powered Boeing XB-39 Superfortress ended its days as a single prototype.
Authored By: Staff Writer | Last Edited: 9/19/2016
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.


[ 1 Units ] :
Boeing Company / Fisher (General Motors)
National flag of United States United States (cancelled)
- Ground Attack
- X-Plane / Developmental
99.02 ft (30.18 m)
141.24 ft (43.05 m)
27.72 ft (8.45 m)
(Showcased structural dimension values pertain to the Boeing XB-39 production model)
Empty Weight:
74,516 lb (33,800 kg)
133,512 lb (60,560 kg)
(Diff: +58,996lb)
(Showcased weight values pertain to the Boeing XB-39 production model)
4 x Allison V-3420-11 W24 liquid-cooled engines developing 2,100 horsepower each.
(Showcased powerplant information pertains to the Boeing XB-39 production model)
Maximum Speed:
404 mph (650 kph; 351 kts)
Service Ceiling:
36,089 feet (11,000 m; 6.84 miles)
Maximum Range:
6,251 miles (10,060 km; 5,432 nm)
1,000 ft/min (305 m/min)
(Showcased performance values pertain to the Boeing XB-39 production model; Compare this aircraft entry against any other in our database)
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.
(Showcased armament details pertain to the Boeing XB-39 production model)
XB-39 - Base Project Designation; single, flyable prototype completed.

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