Following the outbreak of World War II, all pilots were conscripted into the military and there was a ban on civilian aviation. In 1940, the company started air ambulance flights for the military. Following the German invasion of Norway, many of Widerøe's pilots and aircraft were flown to Mjøsa where they served as part of the defence. All civilian aircraft were grounded during the occupation, and German authorities demanded that magnetos and propellers be handed in. The workshop at Bogstad was kept busy with the production of ambulance sleds for the German military. In secret, the company also started building the Hønningstad C-5 Polar ambulance aircraft at Bogstad.