By all accounts, Cecil B. DeMille and John Wayne got along very well during the filming of this movie. (DeMille had considered Wayne for the role of Wild Bill Hickok in Une aventure de Buffalo Bill (1936), which went to Gary Cooper). DeMille admired Wayne's ability to improve his character and he liked him so much that he let him select his own costumes.
Cecil B. DeMille had wanted Errol Flynn to play Stephen Tolliver, but Jack L. Warner refused to loan him out.
During the filming of a fight scene with John Wayne, an accident cost actor Victor Kilian the use of one eye.
Although John Wayne was pleased to have been cast in such an important movie, he was unhappy with his part and once complained he was only there to make Ray Milland look like a real man.
The giant rubber squid used in the underwater battle was donated by the studio to the war effort in 1942. The Japanese had conquered British Malaya and French Indochina, then the sources of most of the world's rubber.