PR Mk X and PR Mk XI (types 387, 365 and 370)

When development started on new Merlin 60 powered Spitfires, both the Mk VII and VIII were to have photo-reconnaissance (PR) variants. On 21 April 1942, an order was placed for 70 aircraft, provisionally designated PR Mk VIII. Based on the revised MK VIII airframe these aircraft were to be powered by Merlin 61s and were assigned serial numbers LV643-681 and LV726-756. The Spitfires bearing these serials were eventually built as Merlin 66 LF Mk VIIIs.

A policy change resulted in the pressurized PR variant of the Mk VII being renamed PR Mk X (type 387). This version followed the PR Mk XI into production and was based on the Mk VII airframe with PR Mk XI wings and camera installation. It had the pressurised Mk VII cockpit, with the Lobelle sliding canopy, and retained the fighter style windscreen with the bullet-proof glass panel. The long thin air intake to the cockpit pressurisation system was fitted under the exhaust stacks on the starboard cowling. The performance was similar to that of the PR XI although the pressurised cockpit meant that this version could stay at altitudes of over 40,000 ft for longer without the exhausting physiological effects experienced by the pilots of unpressurised cockpits. Sixteen Mk Xs were built during April and May 1944, with the first mission being flown on 11 May. All saw limited service in 541 and 542 Squadrons in a high altitude reconnaissance role. Experience with this version led to the development and production of the pressurised version of the PR Mk XIX.

The Mk XI (type 365 standard, type 370 tropicalised) was based on a combination of features from the marks VII, VIII and IX. The Mk XI was the first PR variant to have the option of using two vertically mounted F52 cameras with 36-inch-focal-length lens in the fuselage behind the cockpit. Several other configurations could be fitted, depending on mission requirements; the X Type installation, for example had two vertically mounted F24s with 14-inch lens and an oblique F24 with an 8-inch lens mounted above and facing to port. PR Mk XIs used for tactical reconnaissance had an additional, vertically oriented camera in a fairing under each wing.

Physically the Mk XIs had a deeper nose fairing to accommodate a larger 14.5 gal oil tank and used the unarmoured, wrap-around PRU windscreen. Booster pumps for the wing tanks were fitted and covered by teardrop shaped fairings under the wings. Retractable tailwheels were fitted as standard and the majority of the Mk XIs built had the later large-area pointed rudder. 260 Mk XIs were powered by Merlin 61, 63 or 63A engines, while the remaining 211 used the high-altitude Merlin 70. All of the Merlin 70 and 198 of the Merlin 60 series aircraft were fitted with the Vokes Aero-Vee dust filter in the extended, streamlined carburettor air intake under the nose. All Merlin 60 powered aircraft featured the fuel cooler in the port leading edge wing root. The radio installation was either the VHF TR.1133 or TR.1143 fitted with the "Rebecca" beam approach equipment.

Additional slipper drop tanks could be fitted under the centre-section; in common with the Mk IX these could be 30, 45 or 90 gal capacity and, for the Mk XI, a tank of 170 gal capacity was also available.

Spitfire PR Mk XIs were capable of a top speed of 417 mph (671 km/h) at 24,000 ft (7,300 m) and could cruise at 395 mph (636 km/h) at 32,000 ft (9,800 m). Normally Spitfire XIs cruised between these altitudes although, in an emergency, the aircraft could climb to 44,000 ft (13,000 m) However, pilots could not withstand such altitudes for long in a non-pressurised cockpit without suffering from serious physiological effects.

At first, production of the PR Mk XI was delayed because development of the Mk VII and VIII series was delayed. As a result, it was decided to base the Merlin 60 powered PR aircraft on the MK IX airframe. Production was further threatened because of a dispute over RAF PR doctrine: in early 1943, because the new PR Spitfire was delayed, the Air Ministry proposed that all PR units be converted to de Havilland Mosquitoes. After further analysis the Air Staff agreed that Mosquitoes could perform 90% of PR missions so the Spitfire production for only 10% of PR units was mooted. Air Vice Marshal John Slessor, head of Coastal Command pointed out that the Spitfire was smaller than the Mosquito, used half the number of Merlin engines and was faster, more manoeuvrable and quieter and, therefore production should be increased, not reduced. As a result, the Air Staff decided that PR Mk XI production should be accelerated at the cost of the fighters.

The first Mk XIs were built in November 1942: from April 1944 production ran concurrently with the PR Mk XIX before ending in December 1944, when they were phased out in favour of the Mk XIX. In total 471 Mk XI were built by Supermarine.