Authored By Dan Alex (Updated: 8/4/2016):
The British RAF need for more Curtiss P-40 production fighters led them to consider North American Aviation's manufacturing capabilities. North American designers Edgar Schmued and Raymond Rice, seeing this as an opportunity to market a new fighter altogether, seized the moment and produced a design for British review. The design was accepted under a new 1940 specification which required a heavily-armed fighter of considerable speed with the capability to operate at high altitude - all this with a flying prototype to be made available within a 120 day window. Development ensued and inevitably produced the NA-73X prototype within the allotted timeframe (some sources state the aircraft was completed in just 102 days whilst others say as many as 117 days). The NA-73X took to the skies on October 26th, 1940 with its 1,100 horsepower Allison V-1710-F3R inline engine and showed off its tremendous potential. The design was accepted by the British as the "Mustang" becoming an initial production model batch of 320 Mustang I's.
Mustang I's were first flown by British pilots on May 1, 1941 - these with 1,100 horsepower Allison V-1710-39 inline engines. As tactical reconnaissance platforms, these aircraft were modestly-armed with 4 x 12.7mm machine guns. The type exhibited good response and its performance at low level was exemplary, outmatching even that of the fabled Spitfire Mark V's. Though the design proved quite functional, it was soon found that performance capabilities of the system dropped off significantly at altitudes higher than 15,000 feet. As a result, British Army Cooperation Squadrons were assigned the type and utilized them in both low-level reconnaissance and high-speed ground attack roles with its primary function being the former. The first Mustang I mission was accomplished on May 10th, 1942 with No. 26 Squadron. These Mustangs successfully strafed aircraft hangars at Berck sur Mer in German-held French territory. As more and more Mustangs became available, the aircraft would eventually field some 14 total Allied squadrons by the end of 1942 - 10 RAF, 3 RCAF and 1 Polish air group. Most early production Mustangs went to Britain as Mustang Mk.IA (4 x 20mm cannons) and Mustang Mk.II models, numbering some 620 total combined examples.
The USAAF (United States Army Air Forces) took notice of the aircraft and received two evaluation models (from the aforementioned British 620 order total) with the designation of XP-51. The type excelled in tests but the USAAF passed on an order commitment at the time. It was not until General Hap Arnold intervened that no fewer than 55 British-bound Mustangs were reserved for use in American service. These Mustangs became photo-reconnaissance models designated as F-6A and served with the 111th and 154th Observation Squadrons. These squadrons would be the first Mustang operators for the USAAF and see deployment to North Africa in early 1943.
Purchases of USAAF XP-51's full production models began with an initial block of 150 base model P-51's in March of 1942 (note that there was no model letter assigned to these). Aircraft were armed with 4 x 20mm cannons and were utilized for their inherently good low-level operational qualities thanks to their excellent airframes and under-performing Allison engines. These early-form Mustangs were utilized in the Southeast Asia Theater where most of their action took place at these optimal low altitudes.
The A-36 Apache (also unofficially known as "Invader") represented a dedicated ground-attack / dive-bomber version of the P-51 and was ordered by the USAAF in April of 1942. Apaches were armed with 6 x 12.7mm machine guns, with two to a wing and two in the upper nose portion of the fuselage. Underwing bomb racks also complimented the offensive punch. The system - with their Allison engines - first flew in September of 1942, production eventually totaling some 500 examples. Apaches were assigned to the 27th and 86th Bombardment Groups (Dive) and the first A-36A models were put into action in June of 1943. The 311th Fighter Bomber Group stationed in India also took deliveries of the type.
P-51 base models were similar to these Apache aircraft and were also utilized in the low-level ground attack role, though these were built with 4 x 20mm Hispano-Suiza long-barrel cannons instead of machine guns and underwing bomb shackles for bombs. The similar P-51A models represented a total of 310 examples and were fitted with 4 x 12.7mm machine guns instead of cannons.
Despite its usefulness as a low-level intruder, the Mustang had not lived to the high-altitude specifications originally laid out in the 1940 British requirement. As such, Britain and the United States individually began testing the airframe out with the Rolls-Royce Merlin engine common to the superb Supermarine Spitfire (along with a new four-blade propeller). Results of these tests yielded tremendous performance gains to the extent that the Mustang was clocked at 441 miles per hour at nearly 30,000 feet -100 miles per hour faster than the preceding Mustang production models could ever hope to achieve. These new Merlin-powered Mustangs emerged as the Mustang Mk.III (RAF use), P-51B and the P-51C (the major difference for the American models being in place of origin - P-51B's were constructed at the Inglewood, California plant whilst the P-51C's at facilities in Dallas, Texas). Deliveries began to the 354th Fighter Group in December of 1943. Reconnaissance versions of the P-51C's were appropriately designated as F-6C models.
Having the benefit of seeing design work and production through an on-going war, changes relayed from pilots with operational experience of Mustangs could easily be incorporated in future Mustang models. Such was the result with the development of the P-51D, the most definitive in the Mustang series as a whole - inevitably giving the Mustang its classic warbird appearance. Visibility out of the "razorback", framed cockpit was noted as inadequate for the rigors of dogfighting. This was addressed with the implementation of the "tear drop" canopy (sometimes referred to as the "Bubble-Top" or "Bubble" canopy) and instantly allowed for near 360 degrees of visual awareness from the cockpit. With the loss of the razorback upper portion of the fuselage, the fuselage itself was cut down at the rear. Armament now increased to 6 x 12.7mm heavy caliber air-cooled machine guns (3 to a wing) with an improved and simplified ammunition feed system to help iron out a consistent jamming issue. The P-51D incorporated the new K-14 gunsight as well, in an effort to help improve gunnery accuracy. Power was derived from the Packard-produced Merlin inline engine of 1,590 horsepower. P-51D models were produced in two major batches as P-51D-NA of which 6,502 were produced from Block 1 through Block 30 with the new bubble canopy and the P-51D-NT of which 1,454 examples were produced from Block 5 through Block 30 with 6 x 12.7mm machine guns. The P-51D also existed as a 2-seat dual-control trainer and numbered 10 examples in the form of the TP-51D-NT (TP-51D-NT). A reconnaissance platform existed as the F-6D. RAF P-51D models were Mustang Mk.IV's.
The XP-51F (the P-51E designation was reserved but never used) was produced in three examples as a "lightened" light weight test model. This led to two of the aircraft being fitted with a new engine and becoming the XP-51G. Ultimately, both of these designs led to the P-51H production model.
The P-51H (P-51H-NA) appeared as a "lightened" Mustang - proving some 1,000lbs lighter than the P-51D - and improved the overall top overall speed an astounding 487 miles per hour. This particular model never saw combat due to the end of the war, though production of the type had begun before the cessation of hostilities, totaling 555 examples. Two examples of the XP-51J model followed, these based on the XP-51F with a new engine.
The P-51K (P-51K-NT), of which 1,337 were produced, represented an "improved" D-model and fitted with an Aeroproducts propeller. Production of this model encompassed Blocks 1 through 15. A reconnaissance version of this model existed in the F-6K.
P-51L (P-51L-NA) became a single example model and represented an "improved" H-model with a new engine. Similarly, the P-51M (P-51M-NT) existed as one example based on the H-model and fitted with a different engine.
Australia became just the second Mustang producer, building the aircraft under license in the late 1940's. These Mustangs were designated simply as Mustang Mk.20, Mustang Mk.21, Mustang Mk.22, Mustang Mk.23 and Mustang Mk.24.
The type did, in one other Mustang form, fight on in Korea - this becoming the F-82 Twin Mustang. The system melded two P-51 airframes to one wing assembly with 6 x 12.7mm machine guns mounted in the center wing chord. A rectangular tailplane joined the two airframes at the rear. Each Mustang fuselage retained its respective cockpit positions with dual controls with a pilot manning the primary portside cockpit and a pilot / navigator in the starboard cockpit. The Twin Mustang concept originated in 1943 with the design being acted on in January of 1944. The system was intended for use as a long-range escort fighter in the Pacific Theater during World War 2 but the end of the war canceled the initial order of 500, leaving just 20 operational production models complete. 1947 brought about renewed interest in the design, this time as a night-fighter and the system went into production once more, just in time for use in the Korean War.