The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II, the Korean War and other conflicts. The Mustang was designed in 1940 by North American Aviation (NAA) in response to a requirement of the British Purchasing Commission. The Purchasing Commission approached North American Aviation to build Curtiss P-40 fighters under license for the Royal Air Force (RAF). Rather than build an old design from another company, North American Aviation proposed the design and production of a more modern fighter. The prototype NA-73X airframe was rolled out on 9 September 1940, 102 days after the contract was signed, and first flew on 26 October.
The Mustang was originally designed to use the Allison V-1710 engine, which, in its earlier variants, had limited high-altitude performance. It was first flown operationally by the RAF as a tactical-reconnaissance aircraft and fighter-bomber (Mustang Mk I). The addition of the Rolls-Royce Merlin to the P-51B/C model transformed the Mustang's performance at altitudes above 15,000 ft, allowing the aircraft to compete with Luftwaffe's fighters.[nb 1] The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the Rolls-Royce Merlin 66 two-stage two-speed supercharged engine, and was armed with six .50 caliber (12.7 mm) M2/AN Browning machine guns.
From late 1943, P-51Bs (supplemented by P-51Ds from mid-1944) were used by the USAAF's Eighth Air Force to escort bombers in raids over Germany, while the RAF's Second Tactical Air Force and the USAAF's Ninth Air Force used the Merlin-powered Mustangs as fighter-bombers, roles in which the Mustang helped ensure Allied air superiority in 1944. The P-51 was also used by Allied air forces in the North African, Mediterranean, Italian and Pacific theaters. During World War II, Mustang pilots claimed to have destroyed 4,950 enemy aircraft.[nb 2]
At the start of the Korean War, the Mustang was the main fighter of the United Nations until jet fighters, including the F-86, took over this role; the Mustang then became a specialized fighter-bomber. Despite the advent of jet fighters, the Mustang remained in service with some air forces until the early 1980s. After the Korean War, Mustangs became popular civilian warbird and air racing aircraft.
Design and development
North American NA-73X, with short carburetor air intake scoop and the frameless, rounded windscreen. On the production Mustang Mk Is the latter was replaced with a three-piece unit incorporating a bullet-resistant windscreen
In April 1940 the British government established a purchasing commission in the United States, headed by Sir Henry Self. Self was given overall responsibility for Royal Air Force (RAF) production and research and development, and also served with Sir Wilfrid Freeman, the Air Member for Development and Production. Self also sat on the British Air Council Sub-committee on Supply (or "Supply Committee") and one of his tasks was to organize the manufacturing and supply of American fighter aircraft for the RAF. At the time, the choice was very limited, as no U.S. aircraft then in production or flying met European standards, with only the Curtiss P-40 Tomahawk coming close. The Curtiss-Wright plant was running at capacity, so P-40s were in short supply.
North American Aviation (NAA) was already supplying its Harvard trainer to the RAF, but was otherwise underutilized. NAA President "Dutch" Kindelberger approached Self to sell a new medium bomber, the B-25 Mitchell. Instead, Self asked if NAA could manufacture the Tomahawk (the British designation for early models of the P-40) under license from Curtiss. Kindelberger said NAA could have a better aircraft with the same engine in the air sooner than establishing a production line for the P-40. The Commission stipulated armament of four .303 in (7.7 mm) machine guns, the Allison V-1710 liquid-cooled engine (as used on the Tomahawk), a unit cost of no more than $40,000, and delivery of the first production aircraft by January 1941. In March 1940, 320 aircraft were ordered by Sir Wilfred Freeman who had become the executive head of the Ministry of Aircraft Production (MAP), and the contract was promulgated on 24 April.
The NA-73X, which was designed by a team led by lead engineer Edgar Schmued, followed the best conventional practice of the era, but included several new features.[nb 3] One was a wing designed using laminar flow airfoils which were developed co-operatively by North American Aviation and the National Advisory Committee for Aeronautics (NACA). These airfoils generated very low drag at high speeds. During the development of the NA-73X, a wind tunnel test of two wings, one using NACA 5-digit airfoils and the other using the new NAA/NACA 45–100 airfoils, was performed in the University of Washington Kirsten Wind Tunnel. The results of this test showed the superiority of the wing designed with the NAA/NACA 45–100 airfoils.[nb 4]
, one of two Mustang Mk Is handed over to the USAAC for testing
The other feature was a new radiator design that exploited the "Meredith Effect", in which heated air exited the radiator as a slight amount of jet thrust. Because NAA lacked a suitable wind tunnel to test this feature, it used the GALCIT 10 ft (3.0 m) wind tunnel at the California Institute of Technology. This led to some controversy over whether the Mustang's cooling system aerodynamics were developed by NAA's engineer Edgar Schmued or by Curtiss, although NAA had purchased the complete set of P-40 and XP-46 wind tunnel data and flight test reports for US$56,000. The NA-73X was also one of the first aircraft to have a fuselage lofted mathematically using conic sections; this resulted in smooth, low drag surfaces. To aid production, the airframe was divided into five main sections—forward, center, rear fuselage and two wing halves — all of which were fitted with wiring and piping before being joined.
The prototype NA-73X was rolled out in September 1940, just 102 days after the order had been placed; it first flew on 26 October 1940, 149 days into the contract, an uncommonly short gestation period even during the war. With test pilot Vance Breese at the controls the prototype handled well and accommodated an impressive fuel load. The aircraft's two-section, semi-monocoque fuselage was constructed entirely of aluminum to save weight. It was armed with four .30 in (7.62 mm) M1919 Browning machine guns in the wings and two .50 caliber Browning machine guns mounted under the engine and firing through the propeller arc using gun synchronizing gear.[nb 5]
While the United States Army Air Corps (USAAC) could block any sales it considered detrimental to the interests of the US, the NA-73 was considered to be a special case because it had been designed at the behest of the British. In September 1940 a further 300 NA-73s were ordered by the MAP. To ensure uninterrupted delivery Colonel Oliver P. Echols arranged with the Anglo-French Purchasing Commission to deliver the aircraft, and NAA gave two examples (41-038 and 41-039) to the USAAC for evaluation.[nb 6]
United Kingdom operational service
As detailed above, the Mustang was initially developed for the RAF, who were its first users.
As the first Mustangs were built to British requirements, these aircraft used factory numbers and were not P-51s; the order comprised 320 NA-73s, followed by 300 NA-83s, all of which were designated North American Mustang Mark I by the RAF. The first RAF Mustangs supplied under Lend-Lease were 93 P-51s, designated Mark Ia, followed by 50 P-51As used as Mustang Mk IIs. Aircraft supplied to Britain under Lend-Lease were required for accounting purposes to be on the USAAC's books before they could be supplied to Britain, and so Lend-Lease aircraft destined for the RAF were first ordered and paid-for on Britain's behalf by the USAAC.
After the arrival of the initial aircraft in the UK in October 1941, the first Mustang Mk Is entered service in January 1942, the first unit being 26 Squadron RAF. Due to poor high-altitude performance, the Mustangs were used by Army Co-operation Command, rather than Fighter Command, and were used for tactical reconnaissance and ground-attack duties. On 27 July 1942, 16 RAF Mustangs undertook their first long-range reconnaissance mission over Germany. During the Dieppe Raid (19 August 1942) four British and Canadian Mustang squadrons, including 26 Squadron saw action. By 1943–1944, British Mustangs were used extensively to seek out V-1 flying bomb sites. The last RAF Mustang Mk I and Mustang Mk II aircraft were struck off charge in 1945.
The RAF also operated 308 P-51Bs and 636 P-51Cs which were known in RAF service as Mustang Mk IIIs; the first units converted to the type in late 1943 and early 1944. Mustang Mk III units were operational until the end of World War II, though many units had already converted to the Mustang Mk IV (P-51D-NA) and Mk IVa (P-51K) (828 in total, comprising 282 Mk IV and 600 Mk IVa). As all except the earliest aircraft were obtained under Lend-Lease, all Mustang aircraft still on RAF charge at the end of the war were either returned to the USAAF "on paper" or retained by the RAF for scrapping. The last RAF Mustangs were retired in 1947.
U.S. operational service
of 374th Fighter Squadron, an early D-model, without the fin strake
; 75 US gallons (62.5 imp gal; 284 L) drop tanks
are on the wing racks.
Pre-war doctrine was based on the idea "the bomber will always get through". Despite RAF and Luftwaffe experience with daylight bombing, the USAAC still believed in 1942 that tightly packed formations of bombers would have so much firepower that they could fend off fighters on their own. Fighter escort was low-priority, and when an escort fighter was planned in 1941, a heavy fighter with twin engines, such as the Lockheed P-38 Lightning, was considered to be most appropriate. Another school of thought favored a heavily up-armed "gunship" conversion of a strategic bomber. A single-engined high-speed fighter with the range of a bomber was thought to be an engineering impossibility.
Eighth Air Force bomber operations 1942–1943
The 8th Air Force started operations from Britain in August 1942. At first, because of the limited scale of operations, there was no conclusive evidence American doctrine was failing. In the 26 operations flown to the end of 1942, the loss rate had been under 2%.
In January 1943, at the Casablanca Conference, the Allies formulated the Combined Bomber Offensive (CBO) plan for "round-the-clock" bombing – USAAF daytime operations complementing the RAF nighttime raids on industrial centers. In June 1943, the Combined Chiefs of Staff issued the Pointblank Directive to destroy the Luftwaffe's capacity before the planned invasion of Europe, putting the CBO into full implementation. Following this, the 8th Air Force's heavy bombers conducted a series of deep-penetration raids into Germany, beyond the range of escort fighters.
German daytime fighter efforts were, at that time, focused on the Eastern Front and several other distant locations. Initial efforts by the 8th met limited and unorganized resistance, but with every mission the Luftwaffe moved more aircraft to the west and quickly improved their battle direction. The Schweinfurt–Regensburg mission in August lost 60 B-17s of a force of 376, the 14 October attack lost 77 of a force of 291—26% of the attacking force. Losses were so severe that long-range missions were called off.
For the US, the very concept of self-defending bombers was called into question. But instead of abandoning daylight raids and turning to night bombing, as the RAF suggested, they chose other paths; at first it was believed that a bomber with more guns (the Boeing YB-40) would be able to escort the bomber formations, but, when the concept proved to be unsuccessful, thoughts then turned to the Lockheed P-38 Lightning. [nb 7] In early 1943 the USAAF also decided that the Republic P-47 Thunderbolt and P-51B be considered for the role of a smaller escort fighter and, in July a report stated that the P-51B was "the most promising plane" with an endurance of four hours 45 minutes with the standard internal fuel of 184 gallons plus 150 gallons carried externally. In August a P-51B was fitted with an extra internal 85 gallon tank and, although there were problems with longitudinal stability and some compromises in performance with the tank full, it was decided that because the fuel from the fuselage tank would be used during the initial stages of a mission, the fuel tank would be fitted in all Mustangs destined for VIII Fighter Command.
The P-51 Mustang was a solution to the need for an effective bomber escort. It used a common, reliable engine and had internal space for a huge fuel load. With external fuel tanks, it could accompany the bombers from England to Germany and back.
However, the Allison engine in the P-51A had a single-stage supercharger that caused power to drop off rapidly above 15,000 ft. This made it unsuitable for combat at the altitudes USAAF bombers planned to fly at. Following the RAF's initial disappointing experience with the Mustang I (P-51A) Ronald Harker, a test pilot for Rolls-Royce suggested fitting a Merlin 61, as fitted to the Spitfire Mk IX. The Merlin 61 had a two-speed two-stage intercooled supercharger, designed by Stanley Hooker of Rolls-Royce and this gave an increase in horsepower from the Allison's 1,200 to 1,620 (or 1,720 in War Emergency Power) an increase of top speed from 390 mph (628 kph) to 440 mph (708 kph) as well as raising the service ceiling to almost 42,000 ft (12,800 metres).
Initial flights of what was known to Rolls-Royce as the Mustang X were completed at Rolls-Royce's airfield at Hucknall in October 1942 and after urging from the US assistant air attaché in the UK, Thomas Hitchcock Jr., a similar conversion then took place in the US, leading to production of the P-51B beginning at North American's Inglewood California plant in June 1943 and P-51s started to become available to the 8th and 9th Air Forces in the winter of 1943–1944. During the conversion to the two-stage Merlin engine, which was slightly heavier than the single-stage Allison and so moved the aircraft's centre-of-gravity forward, North American's engineers took the opportunity to add a large additional fuselage fuel tank behind the pilot, greatly increasing the aircraft's range over that of the earlier P-51A.
By the time the Pointblank offensive resumed in early 1944, matters had changed. Bomber escort defences were initially layered, using the shorter-range P-38s and P-47s to escort the bombers during the initial stages of the raid before handing over to the P-51 when they turned for home. This provided continuous coverage during the raid. The Mustang was so clearly superior to earlier US designs that the Eighth Air Force began to steadily switch its fighter groups to the Mustang, first exchanging arriving P-47 groups for those of the 9th Air Force using P-51s, then gradually converting its Thunderbolt and Lightning groups. By the end of 1944, 14 of its 15 groups flew the Mustang.
The Luftwaffe's twin-engine heavy fighters brought up to deal with the bombers proved to be easy prey for the Mustangs and had to be quickly withdrawn from combat. The Focke-Wulf Fw 190A, already suffering from poor high-altitude performance, was outperformed by the Mustang at the B-17's altitude, and when laden with heavy bomber-hunting weapons as a replacement for the more vulnerable twin-engined Zerstörer heavy fighters, it suffered heavy losses. The Messerschmitt Bf 109G had comparable performance at high altitudes, but its lightweight airframe was even more greatly affected by increases in armament. The Mustang's much lighter armament, tuned for anti-fighter combat, allowed it to overcome these single-engined opponents.
The Luftwaffe adapted to the U.S. fighters by modifying their tactics. Instead of approaching in small ad-hoc formations, they formed up en masse in front of the bomber's line of approach, and then attacked by making a single pass through the formation. Flying in close formation with the bombers, the P-51s had little time to react before the attackers were already flying out of range.
Fighting the Luftwaffe
Pilots of the all black American 332nd Fighter Group
(the Tuskegee Airmen
) at Ramitelli, Italy. From left, Lt. Dempsey W. Morgran, Lt. Carroll S. Woods, Lt. Robert H. Nelron, Jr., Capt. Andrew D. Turner and Lt. Clarence P. Lester.
Major General James Doolittle, the new commander of the 8th Air Force as 1944 started, ordered fighter pilots in the late northern winter of 1944 to stop flying in formation with the bombers and instead attack the Luftwaffewherever it could be found. The aim was to achieve air supremacy. Mustang groups were sent in before the bombers in a "fighter sweep" in order to intercept German fighters. As a result, the Luftwaffe lost 17% of its fighter pilots in just over a week, and the Allies were able to establish air superiority.
The Luftwaffe' answered with the so-called Gefechtsverband (battle formation). This consisted of a Sturmgruppe of heavily armed and armored Fw 190As escorted by two Begleitgruppen of light fighters, often Bf 109Gs, whose task was to keep the Mustangs away from the Sturmböcke Fw 190As attacking the bombers. This strategy proved to be problematic, as the large German formation took a long time to assemble and was difficult to maneuver. It was often intercepted by the P-51 "fighter sweeps" before it could attack the bombers. However, German attacks against bombers could be effective when they did occur. Heavily-armed and -armoured Fw 190As swept in from astern and pressed their attacks to within 100 yds (90 m).
While not always able to avoid contact with the escorts, the threat of mass attacks and later the "company front" (eight abreast) assaults by armored Sturmgruppe Fw 190s brought an urgency to attacking the Luftwaffewherever it could be found, either in the air or on the ground. Beginning in late February 1944, 8th Air Force fighter units began systematic strafing attacks on German airfields with increasing frequency and intensity throughout the spring with the objective of gaining air supremacy over the Normandy battlefield. In general these were conducted by units returning from escort missions but, beginning in March, many groups also were assigned airfield attacks instead of bomber support. The P-51, particularly with the advent of the K-14 Gyro gunsight and the development of "Clobber Colleges" for the training of fighter pilots in fall 1944, was a decisive element in Allied countermeasures against the Jagdverbände.
The numerical superiority of the USAAF fighters, superb flying characteristics of the P-51, and pilot proficiency helped cripple the Luftwaffe's fighter force. As a result, the fighter threat to US, and later British, bombers was greatly diminished by July 1944. The RAF, long proponents of night bombing for protection, were able to re-open daylight bombing in 1944 as a result of the crippling of the Luftwaffe fighter arm. 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."