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 Tom Howard's special effects team) to look like it was crashing in Norway, when in fact no shooting was done in Norway. For scenes set in Norway, the mountains of Scotland were pressed into service.
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.
Producer Walter Mirisch filmed this in the UK to take advantage of tax breaks, known as the Eady Plan. The movie recouped its entire costs at the British box office, and Mirisch tried to repeat the formula by setting up Oakmount Productions, and produced a slate of medium budget WWII featuring an American star in the lead.
The three-barreled anti-aircraft "Nordenfelt Gun" is a triple-mounted MG151/20 Drilling flak weapon that was also adopted by the Yugoslavian Air Force, as it was very versatile and had effective anti-aircraft capability.
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.
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