If ever it can be said that an engine steered the course of war, it would be the Rolls-Royce Merlin that would be mentioned as the one having done it. The Merlin was the heart of some of the most effective combat aircraft of World War II including the Supermarine Spitfire, de Havilland Mosquito, Hawker Hurricane, Avro Lancaster and, of course, the P51 Mustang.

Rolls-Royce had developed a reputation as a top aircraft engine company during World War I, but during the blossoming of civil aviation in the 1920s had fallen behind other notable British companies. It was aviation racing that brought Rolls-Royce back to the forefront with its R type engine and put Rolls-Royce in a position for serious consideration for RAF contracts.

The Merlin engine began making test flights during 1937. When the first engine was built, Rolls-Royce employed fewer than 7000 people, such was the impact of the engine type on the company that by the end of World War II the company employed more than 55,000. This team combined to produce a total of over 100,000 Merlin engines, a further 60,000 being produced under license by Packard. Packard Merlins found their way not only into the Mustang, but also the P40, Spitfire, Mosquito and Lancaster.

Between 1939 and 1945 the Merlin saw operational service in 19 different types of aircraft. Horsepower ranged from approximately 1000 Hp in early models to over 2000 Hp in the Merlin 66. It was the pairing of the Merlin with the North American Mustang that turned a modestly performing aircraft into one of the greatest fighters of all time.

It was not only the performance which made the Merlin a great engine, but also its reliability, ease of maintenance and ability to take punishment and still maintain power.