The 8th AF Bomb Run that killed Many U-Boat Commanders. (Also shown in Das Boot)

[Chromius]

Here is a link I stumbled across looking for more info on B-17's. It is a small recount of a crewmember when they bombed the sub-pens that day.

http://aafo.com/library/history/B-17/b17part3.htm

[Sailor Steve]

Well, Das Boot had a low-level glide-bombing raid that takes place right at the end of 1941. The real-life raid that destroyed the real U-96 was a high-altitude level bombing raid that took place in 1944.

So the one in the movie is completely fictional, though such raids did occasionally happen.