This is what you are going to need to reproduce the sound:
http://flickr.com/photos/brraveheart.../in/set-60535/ ...LOL Yes, nothing else sounds like it and I don't think the neighbors would appreciate it.
The sound is more a factor of the size of the prop and that the B-17 was a radial engine. Radial engines have a very distinct sound which is the sound that you are probably looking for. I would look for a .wav file of the sound of the engine and propeller.
Your not going to reproduce that beautiful sound with computer fans. By the way, I love the sound of the old warbirds at airshows. There is nothing that sounds better than a low altitude pass by an old 4 radial engine bomber.