Supermarine Spitfire (early Merlin Powered Reconnaissance Variants)

''This entry is about early Merlin powered reconnaissance Spitfires. For details of late Merlin powered reconnaissance Spitfires, see the Supermarine Spitfire (late Merlin Powered Reconnaissance Variants) entry. For details of Griffon powered reconnaissance Spitfires, see the Supermarine Spitfire (Griffon Powered Reconnaissance Variants) entry.''

Before the Second World War, the conventional wisdom was to use converted bomber types for airborne photo reconnaissance. These bombers retained their defensive armament, which was vital since they were unable to avoid interception. It was soon found that modified Bristol Blenheims and Lysanders were easy targets for German fighters and heavy losses were being incurred whenever these aircraft ventured over German territory.

In August 1939, Flying Officer Maurice Longbottom, inspired by Sidney Cotton, filed a memorandum Photographic Reconnaissance of Enemy Territory in War with RAF Headquarters. In the memorandum Longbottom advocated that airborne reconnaissance would be a task better suited to fast, small aircraft which would use their speed and high service ceiling to avoid detection and interception. He proposed the use of Spitfires with the armament and radios removed and replaced with extra fuel and cameras. As a result of a meeting with Air Chief Marshal Hugh Dowding, Air Officer Commanding RAF Fighter Command, two Spitfires N3069 and N3071 were released by RAF Fighter Command and sent to the "Heston Flight", a highly secret reconnaissance unit under the command of Acting Wing Commander Cotton.

These two Spitfires were "Cottonised" by stripping out the armament and radio-transmitter, then, after filling the empty gun ports and all panel lines, the airframe was rubbed down to remove any imperfections. Coats of a special very pale blue-green called Camoutint were applied and polished. Two F24 cameras with five-inch (127 mm) focal length lenses, which could photograph a rectangular area below the aircraft, were installed in the wing space vacated by the inboard guns and their ammunition containers as a stop-gap measure. Heating equipment was installed on all PR Spitfires to stop the cameras from freezing and the lenses from frosting over at altitude. These Spitfires, which later officially became the Spitfire Mk I PR Type A, had a maximum speed of 390 mph. Several of the sub-types which followed were conversions of existing fighter airframes, carried out by the Heston Aircraft Company. The Type D, which was the first specialised ultra long-range version, was the first to require that the work be carried out by Supermarine.

In the Mk I PR Type B (also known as Medium Range [MR]) conversions which followed, the F24 camera lenses were upgraded to an eight-inch (203 mm) focal length, giving images up to a third larger in scale. An extra 29 gal (132 l) fuel tank was installed in the rear fuselage. It had been envisaged that much larger cameras would be installed in the fuselage immediately behind the pilot but at the time RAF engineers believed this would upset the Spitfire's centre of gravity. Cotton was able to demonstrate that by removing lead weights, which had been installed in the extreme rear fuselage to balance the weight of the constant speed propeller units, it was possible to install cameras with longer focal-length lens in the fuselage. The Type B was the first to dispense with the heavy bullet resistant windscreen. Many of these early PR Spitfires were fitted with the Merlin XII engine and Rotol constant-speed propeller with the early, blunt spinner of the Spitfire Mk II.

The Mk I PR Type C carried a total of 144 gal (655 l) of fuel and was the first photo reconnaissance aircraft to reach as far as Kiel. The extra fuel was carried in the tank behind the pilot and in a 30 gal (136 l) blister tank under the port wing, which was counterbalanced by a camera installation in a fairing under the starboard wing. A larger oil tank was installed, necessitating the reshaping of the nose to the distinctive PR Spitfire "chin". This version was also known as the Long Range or LR Spitfire.

The Mk I PR Type D (also called the Extra Super Long Range Spitfire) was the first PR variant that was not a conversion of existing fighter airframes. The Type D carried so much fuel that it was nicknamed "the bowser". The D shaped wing leading edges, ahead of the main spar, proved to be an ideal location for an integral tank. Accordingly, in early 1940, work started on converting the leading edges, from rib four to rib 21, by sealing off the spar, outer ribs and all skin joins allowing 57 gal (259 l) of fuel to be carried in each wing. Because the work was of low priority and with the urgent need for fighters the first two, hand-built prototypes of the PR Type Ds were not available until October. These prototypes also had a 29 gal (132 l) tank in the rear fuselage. An additional 14 gal (63 l) oil tank was fitted in the port wing. The cameras, two vertically mounted F24s with 8 inch (20.3 cm) or 20 inch (50.8 cm) lens or two vertically mounted F8s with 20-inch (510 mm) lens, were located in the rear fuselage. With the full fuel load the center of gravity was so far back the aircraft was difficult to fly until the rear fuselage tank had been emptied. Despite these difficulties the type quickly proved its worth, photographing such long distance targets as Stettin, Marseilles, Trondheim and Toulon.

Once the first two Type Ds, P9551 and P9552 had proven the concept the production aircraft, which were soon renamed PR Mk IV, were modified to increase the leading edge tank capacity to 66.5 gal (302 l) and by omitting the rear fuselage tank. These aircraft were better balanced and had the more powerful Merlin 45 engine as used by the Mk V, along with heated cabins, which were a great comfort to pilots on such long flights. A total of 229 Type Ds were built.

A single Mk I PR Type E N3117 was built to address a requirement for oblique close-ups as opposed to high altitude vertical pictures. This conversion carried an F24 camera in a fairing under each wing. These faced forward, were splayed outwards slightly and aimed downwards at about 15 degrees below the horizontal. A 29 gal (132 l) fuel tank was fitted in the rear fuselage. N3117 proved most useful as it was able to photograph targets under weather conditions that would make high altitude photography impossible and experience with this aircraft resulted in the development of the Type G.

Mk I PR Type F was an interim "super-long-range" version which entered service in July 1940, pending the Type D. The Type F carried a 30 gal fuel tank under each wing, plus a 29 gal tank in the rear fuselage, as well as having an enlarged oil tank under the nose. It was a useful enough improvement that nearly all existing Type Bs and Type Cs were eventually converted to the Type F standard. Operating from East Anglia it was just able to reach, photograph and return from Berlin. 15 of these were based on the Mk V airframe.

The Mk I PR Type G was the first fighter-reconnaissance version and performed a similar low-level tactical role to the Type E. One oblique F24 camera, with either an eight-inch or 14 inch lens, was fitted facing to port, between fuselage frames 13 and 14. Two vertical F24 cameras were also installed in the fuselage. The forward camera, installed below the oblique, could be fitted with a five-inch or an eight-inch lens while the rear camera could be fitted with an eight-inch or a 14-inch lens. A 29 gal (132 l) fuel tank was fitted just behind the pilot. The first PR Gs were converted from Mk I airframes and their Merlin II engines replaced with Merlin 45s. Late PR Gs were converted from Mk V airframes. The Type G was fully armed with 8 × .303" Brownings and retained the armoured windscreen and gunsight.

A feature of most PR Spitfires were the specially modified "Blown" canopies which incorporated large lateral teardrop shaped blisters, allowing the pilots a much clearer view to the rear and below, vital for sighting the cameras. The lateral cameras were aimed by lining up a tiny +, marked on the side of the blister, with a fine black line painted on the port outer aileron. On all unarmed PR conversions the gunsight was replaced by a small camera control box from which the pilot could turn the cameras on, control the time intervals between photos and set the number of exposures.

In 1941, a new system of mark numbers was introduced, independent of those used for the fighter versions. Also, several PR conversions were re-converted to later PR types.
 * The Type C became the PR Mk III.
 * The Type D became the PR Mk IV.
 * The Type E became the PR Mk V.
 * The Type F became the PR Mk VI.
 * The Type G became the PR Mk VII.

PR Mk XIII (Type 367)
The PR Mk XIII was an improvement on the earlier PR Type G with the same camera system but a new engine, the Merlin 32, which was specially rated for low-altitude flight. It carried a light armament of 4 × .303" Browning machine guns. The first prototype Mk XIII was tested in March 1943.

Twenty-six Mk XIIIs were converted from either PR Type G, Mk II or Mk Vs. They were used for low level reconnaissance in preparation for the Normandy landings.