The XB-15 was powered by the same base engine that powered the B-24, the P&W R-1830, but at a much lower power rating than that achieved by the turbocharged units of the Liberator. The wings were re-engined with Wright R-2600s and fitted to the Boeing 314 "Clipper". The XB-19 was to take the Allison V-3420 double engine in later life. The ultimate solution was to go overboard on power, and the B-36 was born.
Part of the B-29's problems were caused by the nacelles which formed catch basins for leaked flammable fluids, waiting for ignition from a red-hot engine. The solution was to either stop the leaks, or add drainage. They put holes in the argument.