It is easy to write off the pre-Merlin Mustang as a mediocre performer, but as Matthew Willis explains, there is far more to the story of the Allison-engined P-51 than first meets the eye.

Between July and October 1942, the Aeroplane and Armament Experimental Establishment at Boscombe Down evaluated North American Mustang Mk.I AL997, and concluded that it ‘should prove extremely efficient for its purpose.’

It was clear by late 1942 that the Mustang’s purpose was not the one that it had been intended for, a conventional day fighter. By 1942 air combat was generally taking place between 22,000 ft and 27,000 ft. While the Mustang had superlative performance below 18,000 ft, above that height it could not compete as the engine lost too much power. There were plenty of circumstances where good low-altitude performance was beneficial though, and the fighter fell almost perfectly into the lap of Army Co-operation Command, which was still casting about for a suitable aeroplane for tactical reconnaissance.

During the Battle of France, Army Co-operation aircrews had discovered to their cost that the Westland Lysander was too slow and vulnerable for tactical reconnaissance, and the Fairey Battle was no better at gathering battlefield intelligence. In the face of enemy air superiority, a fighter-type aircraft was clearly needed. The Mustang arrived at just the right time to replace the Curtiss Tomahawk, which was proving troublesome for a multitude of reasons.

The Mustangs just starting to arrive from America might have gone to Russia, India or the Middle East. Fortunately for the RAF’s fighter-reconnaissance squadrons, the decision was taken in January 1942 to earmark the Mustang Mk. I for this purpose. By the end of the month, over 100 Mustangs were in the UK or en route, and Lockheeds at Speke were working furiously to fit F.24 cameras and carry out a range of other modifications. The camera fit for tactical reconnaissance was an important feature. An oblique attachment had been specially designed, which fitted onto a tray behind the pilot’s head. The camera pointed down and to the side through an aperture in the canopy glazing sealed by a rubber gland.

After Pearl Harbour, the US recognised the low-level reconnaissance potential the Mustang had to offer. The USAAF held back 50 Mustang Mk. IA aircraft from an RAF order and converted them to F-6A standard with K.24 cameras behind the pilots head as with the RAF aircraft, and in the rear fuselage for vertical photography.

The RAF’s Mustang squadrons immediately carved out a role for themselves over occupied Europe, mixing visual and photographic reconnaissance with attacks on targets of opportunity. Two Mustang squadrons were heavily involved in the Dieppe landings, focussing on deep, low-level tactical reconnaissance of road networks around and behind the objective. The Mustang pilots were charged with searching for any signs of enemy forces being moved in to repel the assault. The roads were largely clear, and the reconnaissance missions had little to report, though the Mustang’s first air-to-air kill resulted from the operation, showing that the aircraft was not just a passive contributor to the war effort.

As 1942 passed into 1943, the RAF’s 2 Squadron spent much of the period photographing the Dutch coast for 34 (Photographic Reconnaissance) Wing. Missions known as ‘populars’ were essentially photo-reconnaissance with specific targets, usually requested by the Army. They could be offensive or not, depending on the importance of the target and the urgency of the information required. An experienced unit with the right facilities available could plan and launch a mission within an hour and a half of receiving the request, and the resulting photographs could be processed and assessed rapidly by specialist units, giving almost ‘real time’ intelligence.

Initially the AC Mustang Mk. I carried a single, oblique camera, which was later complemented with a vertical camera. These were automatic in operation (thanks to an electric motor) and photography was initiated by the pilot. The aim was to take a series of photographs at fixed intervals, while flying a fixed course to ensure good overlap between images. Images were often taken of the coastal areas, ensuring an up-to-date understanding of defences.

The Fighter Reconnaissance squadrons were clear that the Allison-powered Mustang was the most suitable aircraft for this function. It had become an accepted principle that they should be equipped with the fastest single seater aircraft at heights up to 5,000 ft, with a range of 600 miles. They had found that the high diving speed and stability of the Mustang made it ideal for low level oblique photography, and its performance allowed it to go toe-to-toe with the best German fighters if required. Its Allison engine proved highly reliable, with bearings that lasted much longer between overhauls than the Rolls Royce or Packard Merlin. Frequently engines that had been damaged by gunfire were able to drag aircraft and pilot home.

The USAAF learned a great deal from RAF operations, and its own tactical reconnaissance with the F-6A and F-6B variants were modelled closely on RAF practice. The F-6As did sterling work in the Mediterranean, as did the F-6Bs in North-West Europe, joining RAF Mustangs in building up a detailed photographic record of the French coast before the invasion in June 1944.

As the Mustang Mk. Is and Mk. IAs were gradually replaced during 1944 and 1945, it was arguably not with aircraft that were better-suited to low-level reconnaissance. Indeed, in some cases, (the Typhoon for example), the replacements were a retrograde step in terms of performance. Unfortunately, supplies of the Allison Mustang had all but run out after production switched to the Merlin-engined version.

Matthew Willis is currently working on a book on the Allison Mustang, its development, service and technical details.

Pre-order The Hush-Kit Book of Warplanes today

Want to see more stories like this: Follow my vapour trail on Twitter: @Hush_kit

Have a look at 10 worst British military aircraft, Su-35 versus Typhoon, 10 Best fighters of World War II , top WVR and BVR fighters of today, an interview with a Super Hornet pilot and a Pacifist’s Guide to Warplanes. Was the Spitfire overrated? Want something more bizarre? The Top Ten fictional aircraft is a fascinating read, as is The Strange Story and The Planet Satellite. The Fashion Versus Aircraft Camo is also a real cracker. Those interested in the Cold Way should read A pilot’s guide to flying and fighting in the Lightning. Those feeling less belligerent may enjoy A pilot’s farewell to the Airbus A340. Looking for something more humorous? Have a look at this F-35 satire and ‘Werner Herzog’s Guide to pusher bi-planes or the Ten most boring aircraft. In the mood for something more offensive? Try the NSFW 10 best looking American airplanes, or the same but for Canadians.