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The working title of this film was Attack by Night. According to a pre-production news item in Hollywood Reporter, the film's setting was changed from France to Norway to exploit public interest in Norway as an occupied country. Other news items in Hollywood Reporter note that Charles Vidor replaced Dorothy Arzner as director after Arzner suffered an attack of pleurisy. According to another news item in Hollywood Reporter, scenes were filmed on Vancouver Island, British Columbia, which doubled for the Norwegian coast. An unidentified contemporary source in the AMPAS Production Files adds that British Columbia Military Units worked as extras in the film's commando scenes.