The Rolls-Royce Merlin engine had a V12 cylinder arrangement.
The Merlin was a liquid-cooled, supercharged V12 aircraft engine developed by Rolls-Royce in the 1930s. Its twelve cylinders were arranged in two banks forming a V, a layout that combined substantial power with a relatively compact shape suitable for fighter aircraft.
The Merlin became especially famous in the Supermarine Spitfire and Hawker Hurricane, as well as in the North American P-51 Mustang and Avro Lancaster. Many Merlin versions were built, so their exact displacement, supercharger equipment, power output and accessories varied. The cylinder arrangement itself remained the engine's defining architectural feature.
The Merlin is sometimes confused with the larger Rolls-Royce Griffon, which was also a liquid-cooled V12. The Griffon displaced more than the Merlin and powered later aircraft, including certain Spitfire variants. Both engines were important, but the Merlin is the V12 most closely associated with the Battle of Britain.