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Top: Boeing XB-38. A B-17 fitted with 12 cylinder Allison V-1710s. 1 built. Flew in May/June of ‘43.
Bottom: Boeing XB-39. A B-29 fitted with experimental 24 cylinder Allison V-3420s (two V-1710s yoked together). 1 built. Flew December of ‘44. Neither went into production because in each case, the modest increase in performance didn’t warrant interrupting production scheduling to make the necessary changes to the assembly lines.
Never heard of the 3420 and stumbled on this rig. I worked there 20 years and never knew GM (Fisher Body no less) built a fighter, let alone with a 3420 in it. A little ungainly looking (front/top heavy) to me, but apparently as good as any prop fighter of the era.
Didnt go anywhere: Fisher P-75 Eagle - Wikipedia
Allison V-3420 - 1930's design, slide rules and pencils. Always amazed at what those folks could do. supercharged and turbocharged.