The idea to put a Merlin engine into the Mustang was first brought up by the British.
In March of '42, Wing Commander Ian Campbell-Orde, commanding officer of the Air Fighting Development Unit, offered an inventation to Rolls Royce service liason test pilot Ronnie Harker to fly a Mustang Mk I (AG422) at Duxford. The following day, Harker sent a letter to the Rolls Royce executives to place the Merlin 61 engine in the Mustang.
In May of '42, the first Mustang (AG518) was sent to Rolls Royce to have the Merlin 61 engine installed and tested but not accepted for installation and testing due to the Mustang selected was Type NA-73 airframe. A Type NA-83 (AM121) airframe was used instead for the Mustang X project.
In June of '42, the USAAF authorized its own Mustang Merlin project after observing the Rolls Royce Mustang X project.
ack-ack
Like I said, debatable. The Brits may have in fact installed one first. I'm not so sure of that though. I read some dates somewhere where an American officer was pushing for the license built Merlin conversion very near the time when Harker made his suggestion.
Maj. Thomas Hitchcock, was an attache of sort I believe and was in England when he made the proposal to U.S. brass. Maybe he got the idea from Harker.
I don't know but my point was, just because the Brits made the first documented suggestion of the conversion doesn't mean that US designers were too stupid to know that the Mustang needed a better engine. And there were better options available to them yet they were either denied access to them, they only provided the Brits with what US officials allowed them to or simply stuck with the engine the Brits wanted in the P40.
I believe they wanted to offer a supercharged engine but were restricted.
As I understood it, the original procurement was paid for by the British too.
I can hear it now, "America doesn't have any competitive fighters in its inventory now, but you're going to build us a better one as a "short cut" and you want us to pay for it? I say old chap, why didn't you build this new fighter sooner and where can I get some of your medicinal inventory?"
Not likely. The Brits already knew the p40 was inferior to their own fighters and that nothing the U.S. had at the time was equal to the Spitfire. Buying the P40 was an act of desperation and they were supposed to believe that a U.S. company could produce a better fighter as a means of saving retooling time?
I'd like to see this "paid for" evidence.