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There was also the B-15, B-16 and B-19 projects that would have provided an alternative if the B-17 program failed for whatever reason.
While innovative, the B-15 just wasn't fast enough at the time, same went for the B-16 project. Without the B-17, perhaps the engines could have been used on the B-15 (the B-16 had Allison V-1710 engines) to enhance it's performance.
If Boeing is to build the Manchester with US engines, they can employ R-2800 there. A Stirling with R-2600 was flown, two prototypes total in Canada? The 'Lancaster minus' seem like a decent plane, though all of them would be hard pressed to do 20000 ft bombing sorties.
The B-24 seem so natural choice.
The R-2800 was offering 100 HP more even in A series, compared with non-restricted Vulture.
I love effective ugly uncool machinery. Probably why I tout the performance in action of the Armstrong Whitworth Whitley bomber, Valentine tank and the Blackburn Skua dive bomber.
The R2800 is not ready in time whereas Twin Wasps, Whirlwinds and Allisons are. Hence the x4 US option. But I still think more B24s would be the default.
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