No list of the greatest aircraft of World War II can be complete without the North American P-51 Mustang, arguably the most important Allied aircraft of the war along with the Spitfire. The P-51 was originally designed for a British specification in April 1940 and the first prototype was built in only 117 days since it was conceived. Although unremarkable at first, when powered by Allison engines, it did incorporate an innovative laminar flow wing which produced less drag. Its full potential was only realized when it was fitted with a Rolls-Royce Merlin engine, which gave it outstanding performance particularly at high altitude. This, combined with its impressive range (it was the only Allied fighter that could fly from England to Berlin and back), turned it into the ideal bomber escort. During the battles of attrition fought in the skies over Germany in 1944, huge numbers of colorful pinup-clad P-51s swung the balance of power decisively in favor of the USAAF by decimating the dwindling Luftwaffe fighter force in the months leading up to D-Day. P-51s were also active in the Pacific theater escorting B-29 bombers into Japan. As a dogfighter, the P-51 benefitted from excellent speed and high-altitude performance, which gave it an edge against the Luftwaffe’s Bf 109s and Fw 190s and made up for its (relatively few) weaknesses which included an unexceptional turn rate and lack of cannons. Some also proved successful against Me 262 jets in the final months of the war. P-51s were also given an extra lease of life during the Korean War where they were used extensively in the first year of the conflict due to their range advantage over the early jets. USAF units were phased out until 1957 and were exported to over 30 countries although wartime use was limited to Canada, China, France, Netherlands, New Zealand, South Africa, Sweden, Australia (where it was license-built by CAC), and of course, by the RAF which was the first service to use it in combat.

The Mustang prototype NA-73 was first flown in 26 October 1940, it was initially ignored by the USAAF and therefore entered service first with the RAF as the Mustang Mk. I. The US began to catch interest after Pearl Harbor and diverted some British units into USAAF service as the F-6A photo-recon aircraft and the A-36A dive bomber before a fighter version was finally introduced as the P-51A. Experiments with Merlin engines led to the high-performance P-51B and C (similar aircraft built in different plants) which corrected many of the high-altitude difficulties of the initial variants. Early versions featured the Packard-built V-1650-3 (based on the Merlin 63) before switching to the more powerful -7 (Merlin 66). With drop tanks they could escort heavy bombers to Berlin and back, a feat no other Allied fighter could match at the time. They also served with distinction in North Africa, Italy and the Far East and were the versions most used by the RAF where their speed made them ideal V-1 flying bomb killers in the final months of the war. The RAF also introduced the blown 'Malcolm Hood' for greater visibility and this too was later adopted by the USAAF. The P-51D introduced the distinctive bubble-top canopy as well as more powerful engines which increased its payload and range, it was the most widely produced variant and undoubtedly one of the most beautiful and recognizable aircraft of all time. Some were used relatively successfully as a ground-attack fighter leading up to and after D-Day but with liquid-cooled engines were more vulnerable to ground fire than P-47s. Virtually identical to the P-51D was the P-51K, built for Commonwealth forces though some ended up in USAAF service as well, meanwhile photo-recon conversions of these two variants were designated F-6D and K respectively. Lastly, the improved P-51H was introduced during the final months of the war, it was lighter, faster and had greater range but was too late to see combat. Post-war, the P-51 was used extensively during the opening stages of the Korean conflict where its long range allowed it to fly from bases in Japan when forward strips were overrun, something the shorter-ranged jets were hard pressed to do. This time, it's role was mainly as a fighter-bomber and the more rugged P-51D (now known as the F-51D following the new USAF designation system) was preferred. In RAF services, other Mustang versions were the Mk. IA (same as the Mk. I but with cannons instead of guns), Mk. II (P-51A), Mk. III (P-51B/C) and Mk. IV (P-51D/K).