The Merlin family of engines powered such famous aircraft such as the Spitfire, Hawker Hurricane, DeHavilland Mosquito, Mustang, and the Lancaster bomber. The Merlin was developed in England in 1936, and used in a prototype Spitfire F39/34. The first production Spitfire in 1939 had a 1030 hp Rolls-Royce Merlin MK II engine.

The Merlin went through continuous development throughout World War Two, ending up with at MK 71. The Merlin series was then superseded by the Griffon series. In early 1941, Rolls-Royce licensed Packard Motors to build Merlin engines. By 1943, the Mustang P51B & C (RAF Mustang III) had a 1520 hp, V1650-3 Packard Merlin engine. In Canada the Packard Merlins were designated Merlin 28 and 29. Standard engine for the P-5ID was the liquid-cooled, l2-cylinder, Packard-built, Rolls-Royce Merlin V-1650-3 or -7 developing 1,400 hp at take-off. The original Mustangs were fitted with the low-altitude rated Allison engine, but as the possibilities of the Mustang as a high-altitude fighter became realized, it was decided to fit a Merlin engine. For this purpose, four Mustang Mark Is were sent to Rolls-Royce for use as development aircraft, AL963, AL975, AM203 and AM208. They had Merlin 61 series engines installed with a frontal radiator, in addition to the normal ventral scoop. The Mustang/Rolls-Royce combination was an instant success and it was adopted as standard for all the Mustang variants. To increase the flow of engines, the Packard Car Company of America built the Merlin under license. The Merlin was fitted with an injection-type carburetor and a two-stage supercharger. The the -3 engine supercharger cut-in at 19,000 feet, and on the -7, between 14,500 and 19,000 feet. The supercharger was automatic, but could be manually over ridden. In order to give the engine an extra burst of power during an emergency, the throttle could be pushed past the gate stop by breaking the safety wire. If used longer than five minutes, there was a risk of severe engine damage. Mustang pilots were left in no doubt when the supercharger cut into the high-blower position, for the aircraft shuddered violently. They had to learn to anticipate the cut-in and reduce throttle. When descending, the change to low-blower took place at about 14,500 feet, and the only indication of the event was a drop in manifold pressure. The Packard Merlin drove either a four-blade Hamilton-Standard Hydromatic or Aeroproducts automatic, constant-speed propeller. Coolant (30/70 ethylene-glycol/water) and oil radiators were installed in the pronounced belly scoop radiator fairing under the fuselage. One weakness of the Merlin was that it could be put out of action by a single bullet, or piece of shrapnel, but this applied to all liquid-cooled engines, and did not detract from the Mustang's all-round capabilities. The aircraft was a welcome sight to the Fortress crews as they plunged deep into German skies during the daylight offensive against the Nazi armament industries.