Even experts on World War II don’t always remember hearing about an American fighter named the XP-78.

But almost every student of the war knows the decision to re-engine the P-51 Mustang with the British-designed Rolls-Royce Merlin was one of the era’s great flashes of genius. The Merlin-powered Mustang is sometimes called the plane that won the war. More or less arbitrarily, a decision was made to change the plane’s designation from XP-78 to XP-51B by the time of its first flight.

Today, conventional wisdom is that the American daylight bombing campaign over Europe succeeded only because U.S. fighters were able to defeat the German fighter force. Moreover, U.S. airmen defeated the Luftwaffe not by bombing its aircraft production, nor by attacking its fuel supply – neither move inflicted a knockout blow – but by killing its pilots.

This happened because of the Mustang, and the Mustang succeeded because of the Merlin.

In a memoir, Mustang ace Col. Clarence E. “Bud” Anderson wrote that the Mustang “went like hell” because “the Merlin had great gobs of power and was equally at home high or low, thanks to its two-stage, two-speed supercharger.”

In an unpublished manuscript, author Jeffrey L. Ethell wrote that the Merlin-powered P-51B initially carried about the same amount of fuel as the P-47 Thunderbolt, but “got 3.3 miles per gallon while the P-47 got less than 1.8.” Mustang ace Brig. Gen. Thomas “Tommy” Hayes said that the Merlin-powered P-51 “had the three qualities you need most if you were going to escort bombers to Berlin – range, range and range.”

Mustang ace Brig. Gen. Thomas “Tommy” Hayes said that the Merlin-powered P-51 “had the three qualities you need most if you were going to escort bombers to Berlin – range, range and range.”

Later in the war, Mustang ace Harrison B. “Bud” Tordoff challenged the vaunted German Messerschmitt Me 262 jet fighter. The jets were making passes at bombers Tordoff was escorting. “Out to the left I saw something coming at us from below,” said Tordoff in a 2008 interview. “Two Me 262s, 8,000 feet below us. I dropped my tanks and dived on them and started shooting from a little far behind and got a single hit on the left jet unit of one of the planes. He began streaming gas. His buddy stayed with him for a while, then peeled off and flew away. A gout of flame came out of his plane and he bailed out, but I didn’t see his chute open.”

 

Merlin Magic

Nothing was inherently wrong with the Allison engine that pulled P-51A Mustangs and A-36 Apaches through the sky (the name Apache would be dropped by July 1942), but the decision to switch to the superb British Merlin redeemed the Mustang’s latent growth potential. In April 1942, Ronald W. Harker, a test pilot with Rolls-Royce, flew the Allison-powered Mustang I and was favorably impressed with its performance up to medium altitudes. Harker also concluded that the Allison-powered Mustang was a disappointing performer at higher altitude.

Others may have been thinking similarly but Harker (1909-1999) is often called “the man who put the Merlin in the Mustang.” He may have been biased as a Rolls-Royce employee, but Harker clearly deserves credit for suggesting the all-important engine change.

Harker told his superiors the Mustang would perform better with Rolls-Royce’s Merlin 61. The “60 series” denoted two-stage, two-speed supercharged versions of the engine. There was strong resistance at the Air Ministry in London. Rolls-Royce’s Ray Dorey, head of the engine flight test section at Hucknall, steered Harker toward Air Chief Marshal Sir Wilfrid Freeman, the air member for production and research, who succumbed to Harker’s persistence. Ultimately, Harker was given a chance to try his idea in an effort of modest scale that some officials believed would fail.

Harker told his superiors the Mustang would perform better with Rolls-Royce’s Merlin 61. The “60 series” denoted two-stage, two-speed supercharged versions of the engine.

In August 1942, the Royal Air Force (RAF) began a program to re-engine just five Mustang airframes with the improved Merlin 65. They were designated the Mustang X. The first flight came on Oct. 13, 1942, with Capt. R.T. Shepherd, Rolls-Royce chief test pilot, at the controls. Trials were completed at the end of December. The re-engined fighter’s performance, especially at high altitude, was spectacular. But the RAF could not proceed with the project because Rolls-Royce’s Merlin production was already allocated to Spitfires, Lancasters, and Mosquitos.

The decision to transform Britain’s Manchester bomber, powered by two Vulture engines, into the Lancaster, propelled by four Merlins, was also largely thanks to Harker. The Manchester performed poorly. The Lancaster became one of the great bombers of the war.

 

American Interest

Lt. Col. Thomas “Tommy” Hitchcock, assistant air attache at the American embassy in London and a renowned polo player, submitted reports to Washington on the merits of the Merlin. Hitchcock quoted a Rolls-Royce finding that the engine change would give the Mustang a maximum speed of 432 miles per hour.

Hitchcock reported the success of early Merlin test flights to the Army Air Forces (AAF) brass and to North American Aviation, Inc., (NAA) officials. Meanwhile, NAA had been fully briefed by Rolls-Royce on the Mustang X project and had started planning their own Merlin-powered Mustang. The U.S. planemaker received authorization to install Merlin 65 engines imported from England into two P-51s that been built for the RAF but not yet delivered.

As in London, there was resistance in Washington. As Hitchcock wrote, “Sired by the English out of an American mother, the Mustang has no parent at Wright Field to appreciate and push its good points.”

As in London, there was resistance in Washington. As Hitchcock wrote, “Sired by the English out of an American mother, the Mustang has no parent at Wright Field to appreciate and push its good points.” Wright Field, Ohio, was the AAF’s flight test facility.

The Packard Motor Car Company had a license to build the Merlin, known in AAF parlance as the V-1650, and was building a version with a single-stage supercharger for the P-40F Warhawk, called the Kittyhawk by the British.

The first of two XP-51B fighters (nee XP-78), retaining the basic airframe but powered by a 1,450-horsepower Packard V-1650-3 Merlin first flew for 45 minutes on Nov. 30, 1942 – just weeks after the British Mustang X – piloted by Bob Chilton. It was found that a chemical reaction between different metals in the cooling system and the glycol coolant was clogging the radiator. A new radiator design and scoop were fitted to the second ship, which proved to be free of the problem and performed well.

The P-51B went into production, with the U.S. Army ordering 400 and Britain more than a thousand, of which just 25 were in due course delivered to the RAF as the Mustang III. North American, which had never previously built a fighter, was now getting more business than it could handle. The company’s Dallas, Texas, plant was chosen as a second outlet to build Mustangs identical to the P-51B, which, in the Lone Star State, were designated P-51C.

With a change to a bubble canopy, the Merlin-powered P-51D became the ultimate Mustang and swept the skies of Europe.

With a change to a bubble canopy, the Merlin-powered P-51D became the ultimate Mustang and swept the skies of Europe. Ethell (1947-1997) wrote that the critical blow to the Luftwaffe was the loss of more than 1,000 fighter pilots between January and April 1944, including 28 with more than 30 kills and eight with more than 100. On a mission to Augsburg, P-51s marked the end of the greatest German threat, the rocket-armed twin-engine fighter, when they shot down 23 of the 77 that were airborne.

Hitchcock saw the beginning of these successes in battle but not the conclusion. Sadly, he was killed in an April 19, 1944, crash while piloting a Merlin-powered Mustang at Salisbury, Wiltshire, England. He was performing a dive test and was unable to pull out.