This film's title is the same code-name for the real World War II spy mission, which was code-named "Crossbow". After the Second World War, because of this movie's title, reference to these events popularized them as being "Operation Crossbow".
In this movie, the character of Hannah Reitsch, played by Barbara Rütting (as Barbara Rueting), was a real-life person. Reitsch was a German aviatrix and at one time Adolf Hitler's own personal pilot. During the Battle of Berlin, Reitsch attempted to persuade Hitler to escape from the city in a small lightweight Fieseler Storch airplane.
Despite receiving top billing, Sophia Loren only appears in a extended cameo role. Producer Carlo Ponti, Loren's husband, believed his wife's popularity in the United States would boost the film's chances at the box office and had her billed accordingly.
The character of Constance Babington Smith in this movie, played by Sylvia Syms, was a real-life person. She was a British military officer who in 1957 after the Second World War wrote the book, 'Air Spy: the Story of Photo Intelligence in World War II'.
Duncan Sandys, a character in this movie played by Richard Johnson, was a real-life person. Sandys was Chairman of the British War Cabinet Committee for the Defence against German rockets and flying bombs. He was the son-in-law of British wartime prime minister, Winston Churchill and was wounded during the Second World War in Norway in 1941 which resulted in him having a permanent limp.
MGM executives did not know the German characters would be speaking German (with subtitles) until they saw the rough cut; they wanted to have the German dialogue dubbed into English, but director Michael Anderson persuaded them to keep the subtitled dialogue.
The title was (briefly) changed by MGM for the US release to "The Great Spy Mission" because the studio thought that having the word "operation" in the title might make people think it was either a medical film or a Robin Hood-type movie, a genre that wasn't doing well at the box office at the time.
For the section of the film where RAF Bomber Command raid the Peenemunde rocket research site the producers used the Avro Lancaster PA474 used by the Cranfield institute of technology. PA474 was painted in 83 squadron colours in the summer of 1964 for the movie. PA474 now flies as part of the RAF Battle of Britain memorial flight.
As Sophia Loren was blacklisted in Arab countries because of her portrayal of a jewish woman who survived concentration camp in Daniel Mann's "Judith" which was shot in Israel , Operation Crossbow was shown in Beirut Lebanon with all the Sophia Loren middle segment sequences removed. The movie poster was changed and her name removed from the top billing. The film was shown at the Cinema Strand and the audience never suspected Sophia Loren was part of this production.
Carlo Ponti and the production company worried that the authentic name chosen for the film was confusing and led to a poor initial showing. This reappraisal led to new names, Code Name: Operation Crossbow and The Great Spy Mission, the name chosen for a re-release in North America. The film was also known as Operazione Crossbow in Italy.