Donald Houston served in the RAF during the Second World War.
Cliff Robertson, an accomplished pilot, wanted to buy one of the Mosquitoes after filming had finished, as he was so impressed with the type. He was not permitted to do this but he later bought a Spitfire Mk IX which he owned until the late 1990s.
Four of the De Havilland Mosquitos seen in this film were airworthy and three could taxi on the ground. The same crash at Abindon Airfield, U.K., shot from a different angle, was used with matte painting (by 'Tommy Howard (IV)''s Special Effects team) to look like it was crashing in Norway. No shooting was done in Norway in fact. For "Norway" scenes, the mountains of Scotland were pressed into service.
Most of the attack on the Fjord at the end of the film was done with 1/48th scale Mosquito models.
One of the Mosquitoes seen in the film (number TA639) is on show in the permanent collection of World War II aircraft at the RAF Museum at Cosford in Shropshire, UK.
The 3-Barreled Anti Aircraft 'Nordenfelt Gun' is a triple mounted MG151/20 Drilling flak weapon that was also adopted by Yogoslavia as it was very versatile and had effective anti aircraft capability.
The film's opening prologue states: "This story is inspired by the exploits of the Royal Air Force and Commonwealth Mosquito air crews during World War II."
The German "fighters" were actually 4-seat Messerschmitt 108 "Taifuns," painted to look like Me-109 fighters.
The Mosquito's used in the film were RS715 Cockpit section only TJ118 Cockpit section only TV959 At Bovingdon airfield, but did not fly in film TW117 Flew in film RS709 Flew in film RS712 Flew in film TA639 Flew in film TA719 Flew in film
The Norwegian underground receives a message in Morse code telling them that "M Day" has been moved up to 0630 hours. It is actually sent in good Morse code, but the entire text of the message reads only "MX" and there is no sign-off.
Three of the airworthy Mosquitoes used in the film were TT35 models (target tug versions of the B35 bomber). These were made to resemble FB MkVI (fighter bomber) versions by painting over the clear perspex nosecones and side windows and fitting dummy machine gun barrels. The fourth airworthy Mosquito was a T3 model with a solid nose which only required the fitting of dummy gun barrels.