The P 51 was originally developed for the RAF as a fighter / interceptor. Initially a disappointment when introduced to the European theatre of war due to poor high altitude performance, the first batch of Mustangs was assigned to British Army Co-operation Command instead of Fighter Command -- where they did excel at low level operations. At one point it was suggested to replace the American Allison engine with the proven Merlin engine, the same that was used in the Spitfire. Following an arrangement with the US Packard Co. to produce the Merlin engine under licence, work advanced quickly to modify the Mustang for the Merlin. The alteration proved to be what was necessary to transform the Mustang to fit the role for which it was intended. Sorely needed by Allied Bomber command to curb unsustainable losses on long range missions, was a fighter that could provide escort to distant targets and home again. The new version of the P51 arrived at a very critical period in the air war and proved to be exactly what was needed to help remedy mounting losses of bomber crews.
Reichsmarschall Hermann Göring, commander of the German Luftwaffe during the war, was quoted as saying, "When I saw Mustangs over Berlin, I knew the jig was up." Whether that is a true quote or not (Goring had previously already seen Spitfires over Berlin), the fact of the matter was that every corner of Germany became vulnerable as Bomber Command now had significant protective cover with the arrival of the long range Mustang.
By 8 May 1945, the 8th, 9th, and 15th Air Force's P-51 groups claimed some 4,950 aircraft shot down (about half of all USAAF claims in the European theater, the most claimed by any Allied fighter in air-to-air combat) and 4,131 destroyed on the ground. Losses were about 2,520 aircraft (not to mention the pilots who were killed, injured, or taken prisoner), – which under war time conditions was better odds than could be expected……acceptable odds at least to higher command.
My model of the P51 Mustang D was flown by Lt. J.E. Young of the 3rd Fighter Squadron Group. This was an Air Commando Unit primarily seeing combat during the Philippines Campaign in 1944-45.