Harker test-flew an RAF Mustang on April 30 1942, and noted that it was 30 MPH faster than the Spitfire Mk V with almost double the range.

The British flight-tested the Mustang X in October and found that the experimental craft significantly out-performed the Allison at high altitudes, generating 200 more horsepower at 20,000 feet and almost 500 more HP at 30,000 feet. While the British research was valuable, the American Merlin Mustang program moved forward almost independently.

On November 30 1942, the XP-51B made its first flight with the new Merlin engine, thoroughly impressing the engineers.