TheMustangRider
Senior Airman
IMO that would be time well spent as the B-24 had a very short development period prior to mass production. In addition to more powerful engines the initial model B-24 might have gotten self sealing fuel tanks, powered gun turrets and controls with better balance. Mass production starts a year later (1942 ILO 1941) but you get a much better aircraft. Perhaps good enough that the very expensive and problem plagued B-29 program gets cancelled.
I see such event very unlikely as Gen. Arnold himself was very involved in the development of the B-29 and pushed its premature mass production with the conviction that victory over Japan through air power alone would give the USAAF its much desired independence from the Army.