In September 1939, the Royal Canadian Air Force (RCAF) had only one operational bomber squadron, RCAF Bomber Squadron Number 3. Because of the lack of aircraft production facilities in Canada, both RCAF Fighter Squadron Number 1 and Bomber Squadron Number 3 were equipped with British-built aircraft.
On 23 April, RCAF Squadron 405 was organized and equipped with twin-engine Wellington bombers. RCAF Squadron 408, established 24 June 1941, flew twin-engine Hampdens. Two more bomber squadrons were formed in December-Squadron 419 (Wellingtons) and Squadron 420 (Hampdens). These squadrons would fly with Royal Air Force planes on bombing raids over Germany and were part of the first 1,000-bomber raid on Cologne (30 May 1942).