My dad was in the AF at the time, and was assigned to the movie as a consultant on AF trivia. He convinced them that the stunt could be done safely, and explained how to do it.
And so it was done. My dad got in trouble, because if the B17 had crashed doing the stunt, the AF would have looked bad. But it didn't crash, and the stunt was a big deal to the movie crowds, so he was forgiven.
https://www.youtube.com/watch?v=TGGynxYbA48
It's still freakin' impressive today.
His other contribution to the movie was the mission briefing wall maps.