The Merlin remains most closely associated with the Spitfire and Hurricane, although the majority of the production run was for the four-engined Avro Lancaster heavy bomber.

One of the most successful aircraft engines of the World War II era, some 50 versions of the Merlin were built by Rolls-Royce in Derby, Crewe and Glasgow, as well as by Ford of Britain at their Trafford Park factory, near Manchester.

Merlin engines remain in Royal Air Force service today with the Battle of Britain Memorial Flight, and power many restored aircraft in private ownership worldwide.

In the early 1930s, Rolls-Royce started planning its future aero-engine development programme and realised there was a need for an engine larger than their 21-litre (1,296 cu in) Kestrel, which was being used with great success in a number of 1930s aircraft.

The Vulture was fitted to the Avro Manchester bomber, but proved unreliable in service and the planned fighter using it – the Hawker Tornado – was cancelled as a result.

[10] Early production Merlins were unreliable: common problems were cylinder head cracking, coolant leaks, and excessive wear to the camshafts and crankshaft main bearings.

The Merlin XX also utilised the two-speed superchargers designed by Rolls-Royce, resulting in increased power at higher altitudes than previous versions.

This substantially improved engine life and reliability, removed the fire hazard of the flammable ethylene glycol, and reduced the oil leaks that had been a problem with the early Merlin I, II and III series.

[21] Ultimately, during tests conducted by Rolls-Royce at Derby, an RM.17.SM (the high altitude version of the Merlin 100-Series) achieved 2,640 hp (1,970 kW) at 36 lb boost (103"Hg) on 150-octane fuel with water injection.

The modified exhaust also conferred an increase in horsepower over the unmodified system of 38 hp (28 kW), resulting in a 5 knot improvement in true air speed.

[29] From Jane's:[30] Most of the Merlin's technical improvements resulted from more efficient superchargers, designed by Stanley Hooker, and the introduction of aviation fuel with increased octane ratings.

[41] In 1940, after receiving a request in March of that year from the Ministry of Aircraft Production for a high-rated (40,000 ft (12,000 m)) Merlin for use as an alternative engine to the turbocharged Hercules VIII used in the prototype high-altitude Vickers Wellington V bomber, Rolls-Royce started experiments on the design of a two-stage supercharger and an engine fitted with this was bench-tested in April 1941, eventually becoming the Merlin 60.

[nb 6] Also considered was an exhaust-driven turbocharger, but although a lower fuel consumption was an advantage, the added weight and the need to add extra ducting for the exhaust flow and waste-gates meant that this option was rejected in favour of the two-stage supercharger.

[48] The use of carburettors was calculated to give a higher specific power output, due to the lower temperature, hence greater density, of the fuel/air mixture compared to injected systems.

However, during the Battle of Britain it was found that if Spitfires or Hurricanes were to pitch nose down into a steep dive, negative g-force (g) produced temporary fuel starvation causing the engine to cut-out momentarily.

[52] At the start of the war, the Merlin I, II and III ran on the then standard 87-octane aviation spirit and could generate just over 1,000 hp (750 kW) from its 27-litre (1,650-cu in) displacement: the maximum boost pressure at which the engine could be run using 87-octane fuel was +6 pounds per square inch (141 kPa; 1.44 atm).

"[55] Small modifications were made to Merlin II and III series engines, allowing an increased (emergency) boost pressure of +12 pounds per square inch (183 kPa; 1.85 atm).

[58] Later versions of the Merlin ran only on 100-octane fuel, and the five-minute combat limitation was raised to +18 pounds per square inch (224 kPa; 2.3 atm).

[68] Hives was an advocate of shadow factories, and, sensing the imminent outbreak of war, pressed ahead with plans to produce the Merlin in sufficient numbers for the rapidly expanding Royal Air Force.

[79] Hives further recommended that a factory be built near Glasgow to take advantage of the abundant local work force and the supply of steel and forgings from Scottish manufacturers.

[80][nb 13] This government-funded and -operated factory was built at Hillington starting in June 1939 with workers moving into the premises in October, one month after the outbreak of war.

[83] Hillingdon required "a great deal of attention from Hives" from when it was producing its first complete engine; it had the highest proportion of unskilled workers in any Rolls-Royce-managed factory”.

Worker absenteeism became a problem after some months due to the physical and mental effects of wartime conditions such as the frequent occupation of air-raid shelters.

[88] The Ford Motor Company was asked to produce Merlins at Trafford Park, Stretford, near Manchester, and building work on a new factory was started in May 1940 on a 118-acre (48 ha) site.

[35] In his autobiography Not much of an Engineer, Sir Stanley Hooker states: "... once the great Ford factory at Manchester started production, Merlins came out like shelling peas ...".

Henry Ford rescinded an initial offer to build the engine in the U.S. in July 1940, and the Packard Motor Car Company was selected to take on the $130,000,000 Merlin order (equivalent to $2.83 billion in 2023 dollars[92]).

In chronological order, the first operational aircraft powered by the Merlin to enter service were the Fairey Battle, Hawker Hurricane, and Supermarine Spitfire.

[117] A Spanish-built version of the Messerschmitt Bf 109 G-2, the 1954 Hispano Aviación HA-1112-M1L Buchon, was built in Hispano's factory in Seville with the Rolls-Royce Merlin 500/45 engine of 1,600 horsepower (1,200 kW) – a fitting powerplant for the last-produced version of the famous Messerschmitt fighter, as the Bf 109 V1 prototype aircraft had been powered by the Rolls-Royce Kestrel V-12 engine in 1935.

[118] The CASA 2.111 was another Spanish-built version of a German aircraft, the Heinkel He 111, that was adapted to use the Merlin after the supply of Junkers Jumo 211F-2 engines ran out at the end of the war.

In England the Shuttleworth Collection owns and operates a Merlin-powered Hawker Sea Hurricane IB and a Supermarine Spitfire VC – Both can be seen flying at home displays throughout the summer months.