Perhaps the most important aircraft never to have flown, the Douglas B-31.
In direct competition with the Boeing B-29, the Douglas contender was never completed. If it had been; the B-29 would have been only a historical footnote, the Boeing C-97 and then the B-377 Stratocruiser would never have been created, without their revenue and workforce available Boeing would never have made the 707. Without the 707, the turbo-prop Constellation would have been produced in large numbers, de Havilland and then Vickers would have started a much lower key British jet age.
Initially designed with the "bug eye" cockpit used for the Mixmasters and early C-74 Globemaster, I've modelled the late-build aircraft with the later Globemaster style "standard" cockpit. The aircraft originally was to have very long 3-bladed props, and the late aircraft is fitted with the B-29 style 4-bladed props.
Thanks to @Eswube, the aircraft are both painted from his B-29 collection, as how those particular aircraft would have looked as B-31's not B-29's.