In another 20 years time, when the peoples of this world enter the 21st century, aviation enthusiasts may still be in heated argument as to whether or not the North American P-51 Mustang was the greatest single-seat fighter to be evolved by any of the combatant nations during World War II. There is, however, no doubt at all that it can be numbered among the half-dozen which will be remembered for as long as men record and discuss the history of aviation, hopefully into an epoch when the world's problems are settled by words and work, rather than by weapons and woe.

One of the small number of aircraft to be conceived, developed, produced and put into wide-scale use all within the six years of the war, the Mustang had its origins in April 1940, when the British Purchasing Commission negotiated with North American Aviation to design and build an advanced fighter for the RAF. This had to meet British specifications and, because of the serious situation in Europe, with German forces already in Denmark and Norway and likely to move towards western Europe at any moment, it was stipulated that a prototype must be completed within 120 days.

This was not quite so wishful as one might think, for North American had already drawn up the outline of a new fighter based on information from air combat in Europe, and the company's design team, headed by Raymond Rice and Edgar Schmued, began immediately to shape the tentative design to fit the British specification. Designated NA-73X by North American, the airframe was completed within 117 days, but the 1,100 hp (820 kW) Allison V-1710-39 engine which was to power it was behind schedule, and it was not until 26 October 1940 that the prototype flew for the first time.

Engineers have for countless years made the comment 'if it looks right, it is right'. This was certainly true of the NA-73X, which at first glance registered as a superb example of aircraft design, and which completed quite rapidly a remarkably trouble-free test programme. On 1 May 1941, just over seven months after the prototype's maiden flight, the first production example was flown. The second production aircraft was despatched to Britain for evaluation by the RAF, arriving during November 1941, and was soon followed by a steady flow from the initial contract for 320 NA-73s placed by the British Purchasing Commission before the prototype's first flight.

No time was wasted by the RAF in making its evaluation of the new fighter, given the designation Mustang Mk I. It needed no sophisticated equipment to measure the capability of this aircraft; any experienced pilot could determine after only a few minutes in the air that it was fast and highly manoeuvrable at low levels, far superior to any other US fighter then extant. The limitations came at higher altitudes, for the power output of the Allison engine fell off rapidly as it climbed, which meant that the Mustang I in that particular form was unsuited for combat operation in Europe. However, its particular attributes promised well for deployment in a tactical reconnaissance role, and the standard armament of four 12.7 mm (0.50 in) and four 7.62 mm (0.30 in) machine-guns meant also that it had potential for ground-attack.

The RAF's Mustang Is were therefore provided with an obliquely-mounted camera, behind the pilot on the port side, and in this form were used to equip No. 2 Squadron of Army Co-operation Command in April 1942, the first operational sortie being flown on 27 July 1942. Three months later these Mustangs demonstrated their long-range potential, the first RAF single-engine fighter based in Britain to cross the German border, during an attack on the Dortmund-Ems Canal. The Mustang I was soon found to be fulfilling a valuable role, eventually equipping no fewer than 23 squadrons of Army Co-operation Command, leading to a new contract for an additional 300 aircraft of this mark, and with only minor modifications.

North American's development and production of their NA-73 design for the UK had to receive the blessing of the US government before it could become ratified. A condition of this approval was the supply of two examples to the USAAC for evaluation, and two aircraft from the first production batch were delivered and given the designation XP-51. Before that, however, the US Army had already contracted for the procurement of 150 additional aircraft for supply to Britain under Lend-Lease, designating these P-51, and these differed from the earlier version by having self-sealing tanks, and four wing-mounted 20 mm cannon in place of the eight machine guns. From this batch 93 were supplied to Britain, becoming designated Mustang IA, 55 went to the USAAF as F-6As, equipped with two K-24 cameras for use in a tactical reconnaissance role, and the remaining two also went to the USAAF with different engines, initially as XP-78s, but later brought into the family as XP-51Bs.

USAAF testing of its two XP-51 prototypes had proved highly successful, but at that time the US Army was satisfactorily committed to a large-scale procurement programme involving the Lockheed P-38 and Republic P-47. Their findings confirmed those of the RAF, and it was decided to procure 500 A-36As, these being P-51s provided with dive brakes and underwing racks, to operate as dive-bombers in a close-support role. Armament of this version, first flown in September 1942, comprised six 12.7 mm (0.50 in) machine guns, and the powerplant consisted of an Allison V-17110-87 engine which was rated at 1,325 hp (988 kW) at 3,000 ft (915 m). These were the first Mustangs to enter operational service with the USAAF, equipping two groups in the Middle East in 1943, and used in support operations during the invasions of Sicily and Italy. At about the same time that the US Army had ordered its A-36As, a second contract had been placed for 310 P-51As with a 1,200 hp (895 kW) V-1710-81 engine, armament of four 12.7 mm (0.50 in) machine guns, and with underwing racks to accommodate up to 1,000 lbs (454 kg) bombs, or two 75 or 150 US gallon (284 or 568 litre) drop tanks. Of the foregoing a single A-36A was supplied to the RAF for evaluation, plus 50 P-51As which became designated Mustang II, and 35 were converted for tactical reconnaissance in USAAF service under the designation F-6B, equipped with two K-24 cameras.

At this point in the Mustang story comes the transition to fulfilment of its design potential, initiated in 1942 soon after the first Mustang Is were received in Britain. In order to provide the all-important performance at high altitude, which was needed for the combat fighter role, it was decided to make experimental installations of Rolls-Royce Merlin 61 and 65 engines in Mustang airframes, and four of the Mk Is were supplied to Rolls-Royce at Hucknall for this purpose. Within six weeks the modifications had been completed and the first tests made, demonstrating such improved performance that the results were communicated immediately to North American. They, following the same lines, installed the 1,430-hp (1 066 kW) US-built Packard Merlin V-1650-3 into two P-51 airframes, these duly becoming the XP-78/XP-51Bs mentioned above. It is worth highlighting here that the Allison V-1710 engine installed in the early P-51s was subject to a rapid loss in power after climbing above a height of about 12,000 ft (3660 m); the Packard Merlin V-1650-3 installed in the XP-51B prototypes, and which had a two-speed two-stage supercharger with intercooler, was rated at 1,400 hp (1044 kW) for take-off, and developed 1,450 hp (1081 kW) at 19,800 ft (6035 m).

Early testing of the XP-51B in September 1942 confirmed the British findings, a maximum speed of 441 mph (170 km/h) being attained at 29,800 ft (9085 m). Rate of climb was better than that which the twin-engine P-38 had been built to achieve, 20,000 ft (6095 m) in six minutes, for one of the first test airframes reached this altitude in 5.9 minutes. The USAAF was suitably impressed, ordering large numbers of Merlin-engined Mustangs. The numbers were so large, indeed, that North American's factory at Inglewood, California, could not cope alone, and a second production line was thus established in a new plant at Dallas, Texas.

Inglewood began production of the new model in the summer of 1943 as the P-51B, and an identical version from Dallas was designated P-51C. Both of these differed from the earlier P-51/-51As in having a strengthened fuselage, new ailerons, and small variations which were specific to the new powerplant. Armament comprised four 12.7 mm (0.50 in) machine guns. A total of 1,988 P-51Bs and 1,750 P-51Cs were built before both production lines turned over to construction of the P-51D, more than 2,100 of this total being powered by the V-1650-7 engine which produced 1,450 hp (1081 kW) for take-off and had a combat rating of 1,695 hp (1264 kW) at 10,300 ft (3140 m). 

The RAF began to receive its first Lend-Lease allocations from this P-51B/-51C production at the beginning of 1944, the first equipping No.19 Squadron at Ford, Sussex, in February 1944. All designated Mustang Ill, they comprised 274 aircraft equivalent to P-51B and 636 to P-51C, and were used extensively by no fewer than 21 RAF squadrons, many of which were deployed with the 2nd Tactical Air Force. All of these as delivered had the original sideways opening cockpit canopy which had been standard on all production aircraft until that time, but for air combat the rear view was totally inadequate, and the Mustang Ills were equipped instead with a modified sliding hood which overcame this shortcoming.

In USAAF service P-51B/-51Cs began to enter service a little earlier than with the RAF, being used operationally by the Sth Air Force in Britain for their first long-range escort mission, against Kiel, on 13 December 1943. By early 1944, and using drop tanks to confer the necessary range, they were regularly accompanying Sth Air Force bombers on daylight missions deep into the German homeland, making the first of many visits to Berlin in March, and becoming operational at about the same time with the 10th Air Force in Burma and the 15th Air Force in Italy. Of the 2,828 P-51B/-51C fighters received by the USAAF, 71 P-51Bs and 20 P-51Cs were modified for the tactical reconnaissance role with the designation F-6C.

A P-51D Mustang of the 478th Fighter Squadron 352nd Fighter Group US Army Air Force. Personal aircraft of Lt. Colonel J. E. Meyer

By then, of course, North American had already become involved in what was to become the major production version, the P-51D, of which 7,956 were built, 6,502 coining from Inglewood alone. They differed from the P-51B/-51Cs by introducing as standard a bubble canopy to provide the pilot with an excellent all- round view, a modified rear fuselage, and an armament of six 12.7 mm (0.50 in) machine guns. Of this version 136 were modified to serve as tactical reconnaissance F-6Ds. Later production aircraft introduced as standard a small dorsal fin to compensate for a loss of rear fuselage profile surface resulting from the cockpit modification, and the last 1,100 produced at Inglewood were equipped to launch 5-in (127-mm) rocket projectiles. P-51Ks followed, these differing only by a change in propeller, and of the 1,500 ordered, 163 were completed as tactical reconnaissance F-6Ks. Of the above versions the RAF was allocated 281 P-51Ds and 594 P-51Ks, all designated Mustang IV.

In 1944 the USAAF had contracted for three XP-51F and two XP-51G versions. These were experiments in lightweight construction, and the opportunity was taken to redesign the airframe and take a new look at the entire project. This resulted in the replacement of the original laminar-flow wing by one of a more advanced low-drag section, its planform changed somewhat, and to reduce drag to a minimum the cockpit canopy was elongated and the bulky oil cooler replaced by a shallower heat exchanger. Unnecessary equipment was deleted and substantial weight reductions were achieved by careful redesign and simplification of the structure, and by the introduction of new lightweight materials, including plastics. The XP-51Fs were powered by 1,695 hp (1264-kW) Packard Merlin V-1650-7 engines, the XP-51Gs by the 1,410-hp (1 424-kW) Rolls-Royce Merlin 145M. Subsequently two similar XP-51J prototypes were ordered, to be powered by an Allison V-1710-119 engine which was rated at 1,720 hp (1283 kW) at 20,700 ft (6310 m), but only one of these was completed.

With the knowledge gained from these lightweight prototypes, North American evolved what was to he the last production version of the Mustang, the P-51H. Powered by the V-1650-9 Packard Merlin, which had a combat rating of 2,218 hp (1654 kW) with water injection at 10,200 ft (3110 m), these proved to be the fastest of all the Mustangs, able to attain 487 mph (784 km/h) at 25,000 ft (7620 m). Generally similar to the XP-51F, production aircraft subsequent to number 13 all had vertical tail surface area increased by the introduction of a taller fin and rudder, and all were of an increased length, had a longer dorsal fin and a shorter bubble canopy. More importantly, they had a 40 per cent weight saving by comparison with the P-51D, making possible enhanced performance.

P-51H production totalled 555 before VJ-Day brought cancellation of the balance of the 2,000 ordered. Also cancelled were 1,700 similar V-1650-11 powered P- 51Ls, and 1,628 P-51Ms, which was the Dallas-built version of the P-51H, and of which only a single example was completed. Of these latter versions the RAF received one XP-51F, one XP-51G and one P-51H for evaluation.

On the grand total of 14,819 Mustangs built, production ended in America, but one other source of supply had originated in early 1944 when 314 P-51Ks had been allocated to the RAAF under Lend-Lease. Before any of these were delivered Commonwealth Aircraft Corporation in Australia had begun to tool up to build P-51Ds under licence, beginning by the assembly of 80 aircraft from imported components. Subsequently, a further 120 were built in Australia, but none of the total of 200 was completed in time to be used operationally before VJ-Day. Commonwealth Aircraft production comprised 80 Mustang Mk 20 (P-51Ds), 26 Mk 21 (V-1650-7 engines, 14 later converted to Mk 22), 67 Mk 23 (Merlin 66 or 70 engines) and 13 Mk 22 for tactical reconnaissance.

Under Lend-Lease 50 P-51Ds were supplied to China, 40 to Netherlands forces in the Pacific theatre, and some USAAF P-Sls were supplied to the AVG in China. In the immediate postwar years P-51s remained in service, particularly with Strategic Air Command, until 1949, and others served for several more years with US Air Reserve and Air National Guard units, being among the first USAF fighters to see action in the Korean War. In the RAF some remained in service with fighter command until 1946, and war surplus P-51s from both the USA and the UK continued to have some years of postwar employment with over 50 air forces.

The P-51 was indeed one of the great fighters of World War II and a true hybrid. As a fighter interceptor it was only outclassed by the Focke-Wulf Fw 190D and Ta 152. It as all planes, had its quirks though. Ditching or gear up landings were extremely dangerous as the underfuselage scoop would dig in the ground or water resulting in a tailover. The P-51 also had a fuel tank under the pilots seat. This beside the obvious fire risk, when full, severely affected the planes centre of gravity impacting heavily on performance. Pilots would often use this tank first even when equipped with droptanks. While the P-47 Thunderbolt could accept severe punishment and battle damage, the P-51 was the opposite. Used extensively in the Ground Attack role, its high stall speed and inability to absorb punishment resulted in more P-51s being lost to ground fire than to enemy fighters. A distinction only the P-51 holds. Many historians also consider the lack of any cannon armament a negative vs. other aircraft. While one cannon round from an German aircraft could bring down a P-51, the P-51 itself often required several bursts meaning more time on target to achieve a kill.  

Nicknames: Fifty One; 'Stang; Peter-Dash-Flash

Specifications (P-51D):

Engine: One 1,695-hp Packard Merlin V-1650-7 piston V-12 engine
Weight: Empty 7,125 lbs., Max Takeoff 12,100 lbs.
Wing Span: 37ft. 0.5in.
Length: 32ft. 9.5in.
Height: 13ft. 8in.

Performance:
Maximum Speed: 437 mph
Ceiling: 41,900 ft.
Range: 1300 miles

Armament: Six 12.7-mm (0.5 inch) wing-mounted machine guns, plus up to two 1,000-lb bombs or six 127-mm (5 inch) rockets.

Number Built: Approximately 15,018 (including ~200 built in Australia)

Number Still Airworthy: Approximately 150