The movie was filmed on a low budget. As a result nearly all the shots of the submarines, ships and the action scenes were stock footage from World War II.
During filming Gary Cooper visited the set to persuade his mistress Patricia Neal to abort their unborn baby. Neal acquiesced to his demands and later expressed overwhelming regret at not giving birth. She became an anti-abortion activist in later years, and converted to Catholicism shortly before her death from lung cancer in 2010. Cooper punched Neal in the face after he saw Kirk Douglas attempt to seduce her, the only time he is known to have hit a woman.
John Wayne was dissatisfied with the finished film, believing it to be overlong and containing too much romance. He was also opposed from the beginning to the casting of Patricia Neal, whom he felt was too young at 24 for the role of his ex-wife.
John Wayne and Patricia Neal did not get along during filming. She was particularly annoyed by his treatment of a gay publicity man. Nearly fourteen years later, however, they worked together on In Harm's Way (1965) where she noted that he had mellowed a lot, possibly because he was seriously ill with lung cancer at the time.
The rescue of the women and children is based upon an incident involving the USS Crevalle. The sub took 40 refugees off Negros Island in the Philippines including women, children and some survivors of the Bataan Death March. The rescue was just a cover story. The real reason the sub was sent there was to retrieve a group of important documents, namely the Japanese "Z plan", found by the local guerrilla unit from the crash of a plane carrying the chief of staff of the Japanese Combined Fleet.