Cool project. I just started gathering data for a B-17. Without getting crazy, the biggest motors would be a 5330/250 (53mm dia. x 30mm long stator, Kv=250 RPM/volt) which will give about 14 pounds peak thrust each. Many mfg's offer a motor this size and range in price from $60 to $270 for Axi. The Axi has about 30% more power and runs cooler.

Choose the prop first. Then find a trainer which is 1/4th the flying weight of the B-24 with as close to the same wing loading as possible, build 1/4th of the power system and fly it.

Dave