If you asked a military historian to name the five most important Allied fighter planes of World War II, you can bet they’d include the North American P-51 Mustang and Supermarine Spitfire on the list.

Those two aircraft were common sights in the skies above the European Theatre, as they were adapted to a range of pursuit, bomber, escort, and reconnaissance roles.

And interestingly enough, both planes shared variations of the same powerplant: the 27 liter, supercharged Rolls-Royce Merlin V12.

Workers at the Rolls-Royce factory assemble the V12 Merlin aircraft engine in 1942. (Image: A Merlin Is Made- the Production of Merlin Engines at a Rolls Royce Factory, 1942 by Richard Stone)

Given the Merlin’s British origins, the engine found its way into plenty of other aircraft in the Royal Air Force as well, namely the Hawker Hurricane and de Havilland Mosquito fighter planes, along with the Avro Lancaster heavy bomber. But Merlins also wound up in American planes beside the P-51, like the Curtiss P-40 Kittyhawk.

With its signature elliptical wings, many consider the Spitfire one of the most beautiful airplanes ever made. And thanks to the Merlin, it sounds pretty good too. (Image: Restored Spitfire CGYQQ Y2K S/N TE294, in the air over Comox BC Canada by Guiness323 | CC BY-SA 4.0)

Meet the Merlin

For starters, the Merlin wasn’t Rolls-Royce’s first foray into aircraft propulsion. It evolved out of a program to improve upon the company’s V12 Kestrel engine originally designed in the 1920s. As with a lot of military development programs, the design started with some established performance goals, specifically an airplane that can maintain a minimum airspeed above 310 mph. After a few years of development, testing, and refinement, the Merlin entered service in 1936 in a British Fairey Battle light bomber.

So what made the Merlin engine so special?

Most experts would probably say its supercharger. In addition to increasing the engine’s overall power output, forced air induction was central to the Merlin’s ability to perform at high altitude. As the Merlin continued to evolve throughout the wartime years, improvements to the supercharger system played a big role in its increased performance and versatility.

Here, workers install the Merlin’s supercharger assembly at the back of the engine sometime around 1942. (Image: A Merlin Is Made- the Production of Merlin Engines at a Rolls Royce Factory, 1942 by Richard Stone)

While the original prototype Merlins made around 700 horsepower, that number was over 1,000 by the time the Merlin entered production in 1936. Near the end of the war in 1945, most Merlins were making closer to 1,800 horsepower. And in certain configurations, in specialized high-boost environments, running high-octane fuels, Merlins could be tuned north of 2,000 horsepower.

Packard Lends a Hand

As the RAF began to realize the Merlin’s potential, word of its capabilities quickly spread across the pond.

The United States wanted some Merlin magic too.

Problem was, Rolls-Royce was already cranking out as many Merlins as it could, and all those engines were quickly snapped up by British aircraft manufacturers eager to get their planes out to the Eastern Front. Besides, with U-boat Wolf Packs patrolling the North Atlantic shipping lanes, the threat of sunken cargo vessels full of new Merlins was daunting.

Thankfully, the beleaguered British were happy to license the Merlin to an American company in an effort to get more Allied airplanes into the skies above Europe. The Packard Automobile Company stepped up to the plate with its factories and manufacturing capabilities.

The P-51 Mustang can credit its signature engine snarl to the Merlin. (Image: P-51 Airplane by Crefollet | CC0 1.0)

The Packard Merlin was dubbed V-1650 and it closely mirrored the Rolls-Royce Merlin evolution, as continually updated versions left the Packard factory while the war raged on. While most of the Rolls-Royce Merlins found homes in British aircraft, the Packard Merlin became the powerplant of choice for the North American P-51 Mustang.

The Rolls-Royce Merlin Legacy

As alluded to above, the Rolls-Royce and Packard Merlins were the go-to powerplants for two of the most prominent Allied fighters of World War II: the Supermarine Spitfire and North American P-51 Mustang.

These planes helped fight back the German Luftwaffe and turn the tide of the war in the European Theatre.

Merlin-powered Hurricanes and Spitfires thwarted the German advance in the Battle of Britain, while P-51 Mustangs escorted long-range heavy bombers like the B-17 during the pivotal daylight bombing campaigns that accelerated the end of the war in Europe.

While credit certainly goes to the brave men and women who fought in and alongside these aircraft, the engineers, mechanics, and designers behind the Merlin engine deserve heartfelt recognition as well.

One More Thing….

Want to hear what four Merlins sound like when powering a 36,000-pound Avro Lancaster heavy bomber?

Of course you do. Check it out below:

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Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or watching a 1972 Corvette overheat. An avid motorcyclist, he spends the rest of his time synchronizing carburetors and cleaning chain lube off his left pant leg.