Work starts on the Coning Tower
The bulk of the film's $15 million budget was spent on constructing U-boats. Specifications for the original Type VII-C U-boat were found at the Chicago Museum of Science and Industry. The plans were taken to the original builder of the subs, who was commissioned to build a full-sized, sea-going replica, their first such assignment since the war ended. A second full-sized model was built for interior filming.