Roger Corman came up with the idea for the film during a visit to the dentist, when he read a magazine article about Dubrovnik. To take his mind off the pain while the dentist was working, he tried to create a story which could be filmed there. After leaving the dentist, he returned to his office and wrote the first treatment for the script.
This movie beat The Dirty Dozen (1967) by being the first film about World War II prisoners recruited to go on an impossible suicide mission in exchange for pardons. "The Dirty Dozen" had the same plot but didn't come out until three years later.
This was Stewart Granger's first low-budget independent film after years of major starring roles, and he had concerns about allowing his character to be less than central to the story. During the filming on the Adriatic Sea, the production crew had already created the smoke for the scene when Granger told Roger Corman that he wouldn't do the scene as written because Edd Byrnes had too much dialogue and Granger's character was of lesser importance. Corman quickly wrote some new dialogue for Granger to satisfy his concerns (though he has declined to state how much of that dialogue made it into the finished film), and filming proceeded before the smoke dissipated.
Director Roger Corman once said of this film: "The odd thing about this movie is that it had a very similar plot to an old western of mine, Five Guns West (1955). It also came out before The Dirty Dozen (1967). I've heard stories that the producers of 'The Dirty Dozen' actually postponed production of their film an entire year because they realized that our story-lines were so similar."
Due to an earthquake in Yugoslavia during filming, the production didn't get all of the Yugoslav soldiers they were promised could be used as extras, along with accompanying military equipment, because everything was needed for rescue and recovery work.