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The P-51 made a low key start to it's operational service but after the installation of the Rolls-Royce Merlin engine the North American P-51 Mustang ended the Second World War as one of, if not, the best Allied fighter, and stayed in service with the United States into the 1950s.

The story of the North American Aviation (NAA) P-51 Mustang began in 1940 when NAA President James J. Kindelberger had approached the British Purchasing Commission to sell NAA's twin engined bomber the B-25 Mitchell. However with the Royal Air Force (RAF) in desperate need for fighter aircraft and the Curtiss P-40 Kittyhawk being the only plane coming close to the specification required for the air war in Europe and even this aircraft was only available in limited numbers. So NAA was asked, under licence from Curtiss-Wright, to produce the P-40. NAA felt, however, that they could have a better aircraft working quicker than it would take a production line to produce the P-40 to be set up. Whilst it was agreed for NAA to supply this new aircraft instead it was required for a prototype to be ready in 120 days.
Although this looked like a tall order NAA already had a design outline for a plane which benefited from information from the air war in Europe. Under the leadership of Raymond Rice and Edgar Schmued the design team set about fitting the new fighters design to the specification laid out by the British. And impressively just 117 days later the prototype airframe designated NA-73X was completed, however due to the 1,100-hp Allison V-1710-39 engine, which was to power the plane, running behind schedule, it wasn't until a couple of months later on the 26th October 1940 when the prototype finally flew for the first time.

In a little under seven months and after a very successful testing programme the first production aircraft flew on the 1st May 1941 and six months later the RAF received the second production NA-73 for evaluation. The NA-73 as with a number of other aircraft used by the British, such as the Hawker Tempest and Fairey Battle, an order had been placed before the prototype had even flown and so more of the aircraft soon followed.

Designated Mustang I by the RAF, initial evaluation showed that at low-level the plane was fast and extremely manoeuvrable and was much better than any other US fighter then available. At higher altitude the performance of the plane suffered as its Allison engine power output fell rapidly as it climbed, meaning that despite the Mustang's promise it wasn't suitable for a combat role in Europe.

With its eight machine-guns comprising four 0.50-in and four 0.303-in and it's superb performance at low-level it was within this role that the Mustang would serve with No.2 Squadron of Army Co-operation Command during April 1942, equipped with obliquely-mounted cameras and on on the 27th July they flew their first operational sortie. Just three months later however and the Mustang showed its potential for long range escort duties when in October 1942 on an attack on the Dortmund-Ems canal it became the first RAF single-engined fighter to cross the German border from its base in Britain, however the performance of the aircraft at higher altitude still needed to be improved, but an order for another 300 aircraft was placed.

One of the conditions to allow North American to supply the RAF with it's NA-73 design was that two aircraft were given to the United States Army Air Corps (USAAC) for evaluation under the designation XP-51. Before these two aircraft were supplied to the USAAC the US Army had already ordered 150 more aircraft to be sent to Britain under the lend-lease programme. These planes featured self-sealing tanks and instead of eight machine-guns featured four 20-mm cannons and were designated P-51 by the US and Mustang IA by the British.

A number of these planes went to the United States Army Air Force (USAAF) designated F-6A and were used for tactical reconnaissance and were fitted with two K-24 cameras. And two examples initially designated XP-78 but later XP-51B were each tested with different engines and they proved very successful and the RAF's findings of superb performance at low-level was confirmed. However the US Army was committed to both the Lockheed P-38 Lightning and Republic P-47 Thunderbolt, nevertheless 500 P-51's modified to have dive breaks and bomb racks to provide close support to ground troops and designated A-36 Apache were ordered. First flown during September 1942 and powered by a Allison V-17110-87 engine with six 0.50-in machine-guns, they were the first Mustang variant to go into operational service with the USAAF. Equipping two groups based in the Middle East during 1943 they also performed support operations during the invasions of Sicily and Italy. As well as ordering the A-36 an order for just over 310 P-51A's had been placed by the US Army with armament of four 0.50-in machine-guns and racks for either 1,000lb of bombs or two of either 75 or 150 US gallon external fuel tanks, this was designated Mustang II by the RAF and powered by a 1,200-hp Allison V-1710-81 engine.

After showing it's ability for potential long-range escort duties into Germany, but lacking high-altitude performance, during 1942 the decision was taken to install Rolls-Royce Merlin 61 and 65 engines in four airframes of the Mustang MK I. Testing showed a much improved performance and North American were informed of the results and a 1,430-hp US-built Packard Merlin V-1650-3 engine was installed into two P-51s, these would be designated XP-78/XP-51B. Tested during September 1942 and achieving a maximum speed of 441 mph and having a better rate of climb than the P-38 Lighting, and confirming the British findings, the USAAF impressed with the performance ordered the Merlin powered Mustang in large numbers.

Due to North American not having the production capacity at their factory in Inglewood, California another planet was built at Dallas, Texas. Despite being the same plane, which featured the following improvements over the P-51 and P-51A, of a strengthened fuselage, improved ailerons and a number of small changes to accommodate the new engine and a armament of four 0.50-in machine-guns, a different designated was given depending on which factory had built the plane. P-51B's were planes built in Inglewood and P-51C's were built at Dallas.

P-51B and P-51C's began to enter operational service with the USAAF in Britain as part of the 8th Air Force. The 13th October 1943 saw this new Mustang complete it's first long-range escort mission when a force of bombers were sent to attack the U-boat yard at Kiel. The P-51 Mustang, with the benefit of external fuel tanks, would provide regular escort for bombers of the 8th Air Force, which included the Boeing B-17 Flying Fortress and Consolidated B-24 Liberator, during their perilous daylight operations deep into the heart of Germany. During March 1944 the Mustang took part in it's first mission to Berlin and started being used operationally by both the 10th and 15th Air Force in Burma and Italy respectively. The RAF also started to receive their lend-lease P-51B/P-51C's, designated Mustang III, around the same time and No.19 Squadron based at Ford, Sussex were the first to receive this new type, which would eventually be used by at least 21 RAF squadrons, many of which were part of the 2nd Tactical Air Force. The RAF modified the original cockpit canopy which opened sideways to a sliding hood design to overcome the poor rear view of the original canopy design.

Development of the Mustang continued and the next production version was the P-51D, with a modified rear fuselage, six 0.50-in machine-guns and a bubble canopy as standard, this was to become the most produced version of the P-51. Later versions of the P-51D had a small dorsal fin added and racks to accommodate 5-in rocket projectiles. A change in propeller lead to a change in designation to P-51K, and both of these planes were designated Mustang IV and MK IVA respectively by the RAF.
During 1944 as part of experiments into lightweight construction the USAAF had ordered three XP-51F's and two XP-51G's as part of this process. A new redesigned airframe was also produced and as a result of an overview of the plane a new low drag section of wing replaced the laminar-flow design, the oil cooler was also replaced with a heat exchanger and the cockpit canopy was stretched to reduce drag further. With a simpler structure and the removal of equipment no longer deemed necessary and the use of new lightweight materials being used a significant reduction in the planes weight was achieved. Both prototypes were powered by different engines with a 1,695-hp Packard Merlin V-1650-7 engine powering the XP-51F and a 1,140-hp Rolls-Royce Merlin 145m powering the XP-51G. Two P-51J prototypes of a similar design were ordered, however only one was finished and this would be powered by a Allison V-1710-119, 1,720-hp engine.

As a result of the prototypes the P-51H appeared powered by the V-1650-0 Packard Merlin, this was to be both the last production Mustang and the fastest with a top speed of 487 mph. Further improvements had been made to the plane and as a result the plane was 40% lighter than the last Mustang in war time service the P-51D.
Another permutation of the P-51 was the P-82 Twin Mustang which was two aircraft merged together, however this didn't see service in World War 2.

A grand total of 14,819 Mustangs were built by the time production ended, seeing service with China, Netherlands and a few were also supplied to the American Volunteer Group (AVG) in China. The end of the war didn't see the end of the Mustang as it remained in service with the US in some capacity until 1951, although under a different designation of F (fighter) - 51. In fact the last Mustang was retired from the Dominican Air Force in 1984.