Trivia

The Germans built several submarine bases on the west coast of France after their occupation of the country in 1940. Enormous submarine pens of reinforced concrete were erected at Bordeaux, Brest, La Pallice, Lorient and Saint-Nazaire. These structures protected the U-Boats from attack and allowed them freer access to the Atlantic Ocean, as the submarines did not have to transverse the English Channel or go around the UK via the North Sea from bases in Germany. As of 2020 these massive constructions are still largely intact and some have even been re-purposed. The pens at La Pallice can be seen in Das Boot (1981) and Indiana Jones and the Raiders of the Lost Ark (1981).

French screen star Jean-Pierre Aumont made his American acting debut in this film in a dual role of "Bertrand Corlay/Captain Pierre Metard," as did Swedish actress Signe Hasso as "Elise."

Like the character he plays in Assignment in Brittany (1943), Aumont fought with the Free French forces in Tunisia, Italy and France. He was awarded both the Legion of Honor and the Croix de Guerre. In fact, after only one more film in Hollywood, The Cross of Lorraine (1943), he returned to fight with the Free French. Even in Hollywood, he supported the war effort, appearing at special screenings of Assignment in Brittany, to help raise funds for the Resistance.

Assignment in Brittany (1943) was adapted from a novel Assignment in Brittany by Helen MacInnes (Boston, 1942). It was serialized in The Saturday Evening Post from May 2 to June 20, 1942, and also became a bestseller when published as a book. When MGM bought the screen rights in February 1942, the novel was titled "Cross Channel."

The tune Corlay whistles (used as an orchestral motif) is the French revolutionary air, "Ça Ira" (It'll be okay), underscoring his commitment to the French Republic.

Final film role for Francis X. Bushman Jr. as an uncredited "German Lieutenant." He was the son of silent movie star Francis X. Bushman and the brother-in-law of director Jack Conway.