Michael Goodliffe, who plays Capt. Stein, was an actual Prisoner of War during WWII. He was captured at Dunkirk and spent the next five years in a German POW camp.
The leather jacket that Frank Sinatra wore in this movie was later worn by Bob Crane in Hogan's Heroes (1965). It was later worn by Greg Kinnear in Auto Focus (2002).
The author of the book, David Westheimer, was a navigator on a B-24 that was shot down over Italy in WWII and he was a POW in Stalag Luft III. Westheimer had witnessed the bombing of Bolzano in 1943 from a box car.
The crew were flown back to Hollywood at great expense when studio head Richard D. Zanuck decided to have the Italian POW camp constructed in California. It would also be used for an episode of Voyage to the Bottom of the Sea (1964).
The young actor John Leyton, who guides Ryan through a series of trap doors and secret passages, also played Willie, one of the 'Tunnel Kings' in The Great Escape (1963) just two years earlier.