It was very good-looking and significantly outperformed the B-17, but the B-17 was doing the job. Ergo, no change. While I love the Allison, there was nothing wrong with the B-17 "as is" and the change to the V-1710 would have resulted in better speed, but not by too much. If you don;t get significant changes in performance ... in a bomber ... then why change? In a fighter, a 25 mph increase might have been worthwhile. In a bomber, especially when the defending fighters can catch either one easily, what is the gain?
I agree with the decision to stick with the Wrights.
The driveshatfs in the XB-42 weren't all that long and were supported by bearings at suitable intervals.
I decry that the V-3420 wasn't really used. It did NOT have the tendency for oil leaks inboard (betweeen the two block) that caused the German He-177 to catch fire so frequently and could have been a very good engine for the U.S.A. .