The story of the Merlin Mustang began with a phone call from Wing Commander Ian Campbell-Orde, CO of ARDU at Duxford, to Ronnie Harker, Rolls-Royce service liaison pilot, inviting him to fly a newly arrived Allison Mustang.
On 30th April 1942 he flew AG422 for thirty minutes. He made a subsequent report to Sir A. F. Sidgreaves, Managing Director, and E. W. Hives, Director and General Works Manager, saying in part,
This aircraft should prove itself a formidable low and mid altitude fighter.
The point which strikes me is that with a powerful and good engine like the Merlin 61, its performance should be outstanding, as it is 35 m.p.h. faster than a Spitfire V at roughly the same power.
Ray Dorey, R-R Manager at Hucknell, suggested Harker go direct to Hives. Following conversation with Hives, Hives immediately phoned Sir Wilfred Freeman, Vice Chief of the Air Staff with the recommendation that an aircraft be provided for conversion.
And the rest is history.
The American conversion to the Merlin came at the behest of the American Ambassador to Britain, John G. Winant, instructing General H. H. Arnold, Commander of the USAAF, to give it 'Priority 1' in the factory. (letter penned by W. Lappin, Hives Personnal Assistant, to Hives). From the very start of the program the Americans were involved.
As an aside, there was at one time a very serious examination made by British authorities of producing the Mustang in Britain.