The submarine used in the opening scenes was a replica built in the Halifax shipyards. The Canadian government, although cooperative in the production, could not spare one of its own submarines, which were then patrolling waters in defense of its borders.
This movie represents one of a select group of a few World War II submarine movies which have won the one single Academy Award. Most of them have won their single Oscar in a technical category, that's just only the one Oscar in either special effects or sound editing. These movies include Crash Dive; The Enemy Below; Torpedo Run and U-571. The non-WW II sub-movie, The Hunt for Red October also won just the one Oscar as did the WW 2 part sub-movie 49th Parallel, but for Best Original Story.
Tony Thomas in his book 'The Great Adventure Films' states that this film " . . . was begun in April of 1940 and took eighteen months to complete. More than two-thirds of it was shot in Canada and required a great deal of travel. The Canadian government assisted the film company by giving facilities and loaning service-men and policeman whenever necessary, but the Royal Canadian Navy declined to allow the use of a submarine in the role of the U-37 since the few they had were actively engaged in real warfare. The film company solved their problem by commissioning a shipyard in Halifax, Nova Scotia, to build them a replica of a German U-Boat."
When releasing the film for distribution in the USA, the Columbia Pictures studio changed the film's title from "49th Parallel" to "The Invaders".
While some people may think that this film was inspired by Franz von Werra's escape from a Canadian POW camp (as portrayed in The One That Got Away), von Werra wasn't sent to Canada until January 1941 and his escape wasn't reported until he got back to Germany in April 1941. This film was written as Powell & Pressburger went to Canada in late 1940 and production was under way by January 1941. Powell & Pressburger wouldn't have heard about von Werra until the film was almost completed.