History
Background
With the success of the Allison-powered Mustang Mk I (an initial P-51 w/ Hispano cannons) in British service, the Royal Air Force sought an improvement in the high-altitude performance of the Mustang. On April 29, 1942, Wing Commander Campbell-Orde invited one of Rolls-Royce’s test pilots, Ronald W. Harker, to test the Mustang aircraft. After flying the Mustang, he was impressed with the Mustang’s performance below 15,000 feet, crediting its maneuverability and low-altitude speeds, and proposed fitting the Mustang with a high-altitude Merlin 61 engine with its two-speed two-stage supercharger. He believed the Mustang with a new engine would improve its performance and outperform Spitfires and Typhoons by a margin.
Rolls-Royce engineers were intrigued by his proposal and immediately converted five Mustang Mk Is into the Mustang Mk X with the Merlin by August 12, 1942. Subsequently, the Merlin 65 series, a medium-altitude engine, was fitted to all Mustang X prototypes. The Mustang Xs featured small chin-type radiators mounted underneath the engine and four-bladed propellers. These Mustang X prototypes alone attested to the title of the first Merlin-powered Mustangs.
On October 13, 1942, the first Mustang X serial AL975 conducted its test flight. A second Mustang X had achieved a maximum speed of 413 mph with a full supercharger and 390 mph with a medium supercharger in November. The high-altitude performance in test flights proved a revolutionary improvement for the Mustangs. It prompted Vice-Chief of the Air Staff, Air Marshal Sir Wilfrid R. Freeman, to lobby vehemently for the production of Merlin-powered Mustangs and call for the handover of the fourth and fifth Mustang Xs to the US Army Air Forces for evaluations. This resolution originated the development of prominent Merlin-engine Mustangs that would ultimately break the back of the Luftwaffe in 1943-1945.
Prototype
Aware of the Royal Air Force’s testing with the Merlin-powered Mustang prototypes, the USAAF had little or no interest in exploring the potential of the P-51A and its marriage with the Merlin engine. The North American Aviation engineers and the USAAF Assistant Air Attaché Major Thomas Hitchcock in Britain received a promising outlook of the Merlin Mustang prototypes. They showed advocacy for incorporating the Merlin engines into the USAAF P-51s.
On July 25, 1942, North American Aviation was authorized to convert two Mustang Mark IAs to Merlin 65 engines imported from England. They were initially designated as XP-78 but soon re-designated as the XP-51B. They were powered by a licensed Packard V-1650-3, mirroring the Merlin 61’s performance. The NAA engineers had to move the carburetor air intake from above to below the nose to accommodate the Merlin engine. They added the intercooler radiator located inside the scoop underneath the rear fuselage. Large internal modifications were made to the radiator system. The airframe was strengthened and installed with new ailerons. A four-bladed Hamilton Standard propeller was fitted in the P-51B.
Finally, in November 1942, the XP-51B flew for the first time, and the flight tests confirmed a major improvement in the performance, which was necessary to compete on equal terms with the Fw-190s and late Bf-109s. The USAAF immediately became interested in the Merlin Mustang project and placed an initial contract for 400 P-51B-1s.
Production
By the end of January 1943, the production of the initial P-51B-1 with V-1650-3 began at the factories in Inglewood, California. To incorporate the heavier Merlin 60 series (350 lb more than the Allison series), the P-51B required its wings to move slightly forward to recenter the aircraft’s center of gravity. The wing racks were eventually cleared to carry bombs and drop tanks.
400 P-51B-1s were built as a total without the installation of an additional 85 US Gal fuel tank behind the pilot’s seat. All the P-51B-1s were painted with the olive drab camouflage off the assembly line. Over 1,980 P-51Bs were built across five different production blocks. The P-51B-7 production block started with the incorporation of an extra fuel tank, which increased the range but also shifted the center of gravity and caused directional instability. Beginning with the P-51B-10 and onwards, it was decided to deliver them in their natural metal finish rather than applying the olive drab camouflage, saving extra cost, weight, and drag.
USAAF Service
The first P-51B-1s flew on May 5, 1943, and the P-51B-1s were now the first Merlin-powered production Mustangs in the USAAF’s inventory. The P-51Bs and P-51Cs arrived in Britain in August and October 1943 for the US air units. They would also reach Italy and China, Burma, India Theater.
On November 4, 1943, the 354th FG was the first Fighter Group to equip the brand-new Merlin-powered Mustangs, known as the Pioneer Mustang Group. It was redeployed to Britain from the US mainland during the winter of 1943-1944.
The 354th FG consisted of the 353rd, 355th, and 356th Fighter Squadrons. It was part of the 9th Air Force, responsible for air-to-ground attacks to support the upcoming invasion of Europe; however, its role was immediately switched to support the bomber operations of the 8th Air Force. They flew their first cross-Channel sweep mission on December 1, 1943, and scored their first victory on a mission to Bremen on December 16, 1943. They would start their first long-distance escort mission in January 1944.
The 357th Fighter Group and 363rd Fighter Group were eventually equipped with the new P-51Bs in late January and February 1944.
The P-51B-1s were quickly phased out from the frontline services and replaced with later and improved P-51B/C and P-51D subvariants. These Merlin-powered Mustangs became the USAAF’s most dominant fighters in the European Theater of Operations during the early spring of 1944.
