James Caan received top billing. Caan plays a Canadian amongst a cast of characters who are predominantly English.

This World War II submarine movie was based on an actual mission, while the Australian videocassette sleeve notes state that this movie is, "Based on classified data released by the British Government."

This war movie is loosely based on a real World War II mission, Operation Source, which was staged during September 1943. Operation Source involved a number of secret attacks on several German battleships, namely the "Lutzow", "Scharnhorst", and "Tirpitz", in northern Norway, using X-class mini submarines.

The fifty-man allied submarine seen at the start of the movie was called the "Gauntlet". The name of the German battleship also seen during the opening and later was called the "Lindendorf".

Operation Source was also the subject of Above Us the Waves (1955).

This film's title refers to the X-class submarine, which was a real World War II midget submarine class built for the Royal Navy during the 1943 and 1944 period. The mini subs were also known as "X-Craft".

While training on the Scottish loch, the diver support vessel has the prefix "K", this means the vessel is registered in the Scottish town of Kirkwall.

The movie utilizes black and white archive footage (such as for parachuting) as part of the story (not as historical reference) and as such, these shots do not match. It is rare for this type of color and black and white intercutting to occur in a full color movie, where the archive footage is not intended to be a historical reference.

Nick Tate is credited as "Leading Seaman X-1". When questioned by Captain Erlich (George Pravda) he gives his name as "George Henry Barlow".

This movie is based on the real-life Chariot human torpedo attacks on the German battleship Tirpitz during World War II, as well as Operation Source, which was a series of attacks by X-class midget submarines on Nazi German warships, the "Tirpitz", "Scharnhorst", and "Lutzow", in Northern Norway during World War II.

The real-life World War II German battleship, the "Tirpitz", was renamed the "Lindendorf" for this movie.

Released in the USA in 1969, this picture was made in 1967.

Location shooting took place in Scotland.

Director William A. Graham worked on the film's original screen story with Writer John C. Champion, but is uncredited for such.