Get yourself a field recorder, an excellent mic, and a blimp and record your own. It only takes about eight hours for each airplane. You can probably talk a Fortress pilot into allowing you to record his plane if you paid for every seat on the plane for eight hours, and he might even work out a situation where you can get the internal sounds too. I think the B17 goes for $1250/seat, the same for the B24, but the B29 will be $4500/seat.
After three years experience you might even have some sounds worth using.
