When he was in command of the 8th Air Force, General Ira Eaker did not believe the bombers needed escort. He thought that the bombers' defensive armament would be protection enough. High rates of losses never deterred him from this belief. Accordingly, Eaker turned down the P-38 Lightnings provided him to protect his bombers. The P-38s, with enough range to escort the bombers to any of their targets, spent time in England with relatively little to do, and many were subsequently sent to Africa.

I don't know if I buy this or not.  I thought a major reason for the design of the P-51 was the dire need for a long-range escourt?  

Of the machines and weapons of war, some played a more important part than others, and indeed, a few played a very critical part. But it was a combination of all of them that resulted in victory. No single weapon system decided the outcome of the entire war. Many credit the Mustang with winning the war in Europe because it had the range to escort the bombers all the way to their targets in the heartland of Germany. <b>But this capability was always there, even before the P-51 became available. The fact that long-range fighters were not used earlier was a tactical decision rather than because such fighters did not exist.</b>

In numerous accounts about the development of the Allison-powered Mustang, it has been stated that its poor performance at high altitudes was a surprise and a disappointment to the British and to the USAAF as well. This simply is not so. The aircraft designers of that day had more than sufficient knowledge of powerplants, and they were capable of determining that the Allison engine and supercharger combination installed in the aircraft would have a drop in performance above 15,000 feet. Clear evidence of this is that two of the P-51s, ordered even before the flight of the first XP-51, were reserved for testing with a Packard-built Rolls Royce Merlin engine. They knew that the Allison-powered Mustangs would be low-Ievel fighters, while the Merlin-powered aircraft would be the high-altitude versions. By comparison, Mustangs with the Allison engine could outperform the Merlin-powered variants below 15,000 feet, but no writer has criticized the P-51B, -C, -D, or -K for having less performance at low altitudes. It has been claimed that the Merlin engine is what allowed the Mustang to reach its potential. If only high-altitude performance is considered, this would be true. But a more correct assessment would be that both the Allison and Merlin-powered versions performed very well at the altitudes where they were designed and intended to operate.