Errol Flynn was criticized for playing heroes in World War II movies. Tony Thomas in his book "Errol Flynn: The Spy Who Never Was" states that Flynn had tried to enlist in every branch of the armed services but was rejected as unfit for service on the grounds of his health--he had a heart condition, tuberculosis, malaria and a back problem. He felt he could contribute to America's war effort by appearing in such films as Edge of Darkness (1943), Northern Pursuit (1943), Dive Bomber (1941), Objective, Burma! (1945) and Uncertain Glory (1944). Reportedly, he was at his most professional and cooperative while working on these movies. Warner Bros. apparently did not discuss the state of his health, as it wished to keep it quiet for fear that it would damage his box-office draw.
Although it seems far-fetched for a German sub to be inland within Canada, German U-boats were known to roam off Canada's east coast during the Second World War, destroying a passenger ferry between Newfoundland and Cape Breton Island and 23 Allied cargo vessels and warships in the St. Lawrence River. In the 1980s the remains of a World War II-era German weather station were also discovered in Labrador.
This film's working title was "To the Last Man". News items in "The Hollywood Reporter" add the following information about the production: Alexis Smith was announced as Errol Flynn's co-star, Jesse L. Lasky was to produce the film and A.I. Bezzerides was to write the screenplay. Technical advisor Alf Engen, a ski champion, also played a Nazi in the film. A 6/11/43 item in the paper states that the set was closed to the public because of high-powered love scenes, but no such scenes were in the completed film. Press releases included in the file on the film at the AMPAS Library state that 275 tons of gypsum were used to manufacture the snow effects and a 250-foot ski run was built on one sound stage. Some footage was also shot on location in Sun Valley, ID. Modern sources note that William Faulkner worked on the script.
The original literary source was the novel "5,000 Trojan Horses", by Leslie T. White, published by World's Work, London, 1943, 114 p.