Apparently at least partially based on a true story of Joel David Kaplan. Although I would have to say the way the portray the actual escape in the film is probably the weakest part of the movie. Bronson lands his helicopter in the middle of the courtyard and the prison guards do not do anything until he starts taking off.

Interestingly I thought that this movie had some of the craziest stuntwork I had seen in a while. When I was reading the trivia about it on another site I was actually a bit surprised that there was no mention of serious injury of death. The stunts include dropping out of a helicopter, jumping out of a fast moving car and ducking under the propeller of a fairly fast moving airplane. Somebody actually made a short one minute clip of them if somebody wants to see them:

https://www.youtube.com/watch?v=Pb2X3mOkFd0