The film's scenario and characters were drawn from a novel by Frederick E. Smith published in 1956, which itself drew on several real Royal Air Force operations.
The original idea was to build a wooden bomber with no guns, but they said build a prototype to shut them up. Of course the prototype outran the Spitfire!
When the Norwegian resistance leader, Royal Norwegian Navy Lieutenant Erik Bergman, travels to Great Britain to report the location of a German V-2 rocket fuel plant, the Royal Air Force's No. 633 Squadron is assigned to destroy it. The squadron is led by Wing Commander Roy Grant, an ex-Eagle Squadron pilot (an American serving in the RAF before the US entered the war).
The plant is in a seemingly impregnable location beneath an overhanging cliff at the end of a long, narrow fjord lined with numerous anti-aircraft guns. The only way to destroy the plant is by collapsing the cliff on top of it, a job for 633 Squadron's fast and manoeuvrable de Havilland Mosquitos. The squadron trains in Scotland, where there are narrow glens similar to the fjord.
They used real Mosquitos, and by the way Cliff Robertson owned a Spitfire!