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In April of 1942, a Rolls-Royce test pilot named Ronnie Harker climbed into an unfamiliar American fighter at RAF Duxford and flew it for just 30 minutes.
What he wrote in his report the next morning would change the course of the air war over Europe.
The North American P-51 Mustang had arrived in Royal Air Force service as a disappointment, an Allison-engined airplane that ran out of breath above 15,000 feet and was sent to strafe trains in northern France while Spitfires fought the Luftwaffe overhead.
Yet under that wrong engine sat one of the most advanced airframes ever built, the first fighter in history to use a laminar flow wing, with a fuselage so aerodynamically clean that it carried twice the fuel of a Spitfire and slipped through the air with almost no resistance.
This is the story of how Harker's short memo, written from the Rolls-Royce flight test establishment at Hucknall on May 1st 1942, set in motion the engine swap that produced the finest piston-engined fighter of the Second World War.
From the British Purchasing Commission's desperate spring of 1940 and Dutch Kindelberger's promise to build a fighter from scratch in 120 days, through Edgar Schmued's design office at Inglewood, the Mustang X conversions led by Witold Challier at Hucknall, Captain Ronald Shepherd's first Merlin Mustang flight on October 13th 1942, Robert Chilton's parallel American prototype just weeks later, and on to the Eighth Air Force escort missions that broke the Luftwaffe day fighter arm in early 1944, this documentary traces the chain of small institutional decisions and quiet engineering judgments that turned an American airframe and a British engine into the aircraft that escorted the bombers all the way to Berlin and back.
Drawing on the verified record from Royal Air Force Boscombe Down test reports, the Rolls-Royce archives at Derby, the United States Strategic Bombing Survey interrogations, and the published memoirs of P-51 aces Bud Anderson and Tommy Hayes, this is the human story behind the most famous fighter of the war, told through the names and dates and inconvenient details that the standard narratives tend to lose.
