"Lux Radio Theater" broadcast a 60 minute radio adaptation of the movie on Februray 12, 1945 with Ingrid Bergman, Gary Cooper and Akim Tamiroff reprising their film roles.

Vera Zorina was to play Maria. When she was replaced by Ingrid Bergman, she threatened to sue Paramount, so they gave her a cash settlement.

Ingrid Bergman's first color film.

Ernest Hemingway had Ingrid Bergman in mind as "Maria" while he was writing the novel "For Whom the Bell Tolls".

Ernest Hemingway insisted that Gary Cooper and Ingrid Bergman star in the film, despite the fact that Vera Zorina had already been cast as MarĂ­a and her hair had been cropped.

Ernest Hemingway's novel "For Whom the Bell Tolls" was a 1940 best-seller and reportedly was sold to Paramount Pictures for $100,000.

According to Gary Cooper's daughter, Ernest Hemingway had Cooper in mind for the role of Robert Jordan even before he wrote the original novel.

At the film's conclusion, Gary Cooper's horse falls and breaks its leg. The only horse the crew could get to do the stunt was brown, but Cooper's horse throughout the film was gray. Rather than re-shoot much of the film, Cooper's brown stunt horse was painted gray.

It took 24 weeks to shoot the film (July-October 1942). The first 12 weeks were shot at Sonora Pass in the Sierra Nevada, the last 12 weeks were shot at the Paramount Studio in California.

One of over 700 Paramount productions, filmed between 1929 and 1949, which were sold to MCA/Universal in 1958 for television distribution, and have been owned and controlled by Universal ever since.

Only a handful of Spanish actors were used in the film.

The book (and movie's title) is taken from John Donne's "Meditation XVII" from 1624: ..."No man is an island, entire of itself... any man's death diminishes me, because I am involved in mankind; and therefore never send to know for whom the bell tolls; it tolls for thee."

This film saved the famous love song "As Time Goes By" from being removed from Casablanca. Ingrid Bergman began filming this movie immediately after completing "Casablanca". For this role, her hair was cut short. Meanwhile, for "Casablanca", Warner Brothers wanted to substitute another song for "As Time Goes By" and re-shoot some scenes with Bergman. However, since her hair had been cut, there would be a problem with continuity (even if Bergman wore a wig), so the idea was dropped.

When Ernest Hemingway told Ingrid Bergman she would have to cut off her hair for the role of Maria, she shot back, "To get that part, I'd cut my head off!" She would rehearse tirelessly until all hours of the night, begging to repeat a scene long after the director was satisfied.

Writer Dudley Nichols depoliticized the screenplay, removing all references to Gen. Francisco Franco, loyalists and Falangists. However, he did keep in one prophetic comment about how Germany and Italy were using Spain as target practice.