Stronger undercarriage legs were raked 2 inches (5.08 cm) forward, making the Spitfire more stable on the ground and reducing the likelihood of the aircraft tipping onto its nose.

[2] These were specifically made for the Photo-Reconnaissance Spitfires, including the PR XIX; no armament was fitted and the D-shaped leading edges of the wings ahead of the main spar, were converted into integral fuel tanks, each carrying 66 gallons.

As the Spitfire gained more power and was able to fly at greater speeds the risk of aileron reversal was increasing so the Supermarine design team set about redesigning the wings to counter this possibility.

The Mark numbers used in the aircraft designations did not necessarily indicate a chronological order; for example, the Mk IX was a stopgap measure brought into production before the Mks VII and VIII.

[11] On 4 December 1939, the Supermarine design staff produced a brochure which mooted the idea of converting the Spitfire to use the Rolls-Royce Griffon engine.

However, constant problems with the development of the Griffon meant that the decision to proceed with building a Spitfire with this engine didn't come to fruition until 1942, with the successful flight trials of the Mk IV.

The lower thrust line and larger capacity of the new engine meant that the contours of the engine cowling were completely changed, with more prominent blisters over the cylinder heads, plus a third tear-drop shaped blister on the upper forward cowling to clear the magneto, and a deeper curve down to the spinner, which was much longer than previous types.

[14] Jeffrey Quill, Supermarine's chief test pilot, was the first to fly the Mk IV/Mk XII prototype DP845, ... there was somewhat less ground clearance, resulting in a slight reduction in propeller diameter; the power available for take-off was much greater; and the engine RPM were lower than in the Merlin.

Handling, however, was considered to be better than previous Spitfire marks, and the clipped wings conferred excellent manoeuvrability through enhanced aileron response.

[17] At low altitude it was one of the fastest aircraft in the world; in one speed trial, held at Farnborough in July 1942 DP485 (now referred to as the Mk XII) piloted by Jeffrey Quill raced ahead of a Hawker Typhoon and a captured Focke-Wulf Fw 190, contrary to expectations.

The Spitfire was to be a sort of datum pacemaker – 'Mr Average Contemporary Fighter' – and its job would be to come in last, the real excitement of the proceedings being by how much it would be beaten by the Fw 190 and the Typhoon, and which of these two bright stars would beat the other and by how much.

Outside on the tarmac at Worthy Down stood the inoffensive-looking but highly potent DP485 ... All went according to plan until, when we were about halfway between Odiham and Farnborough and going flat out, I was beginning to overhaul the Fw 190 and the Typhoon.

[19] The Mk XII's speed advantage at lower altitudes again became useful near the end of its front line service in summer 1944, when its pilots were credited with 82.5 V-1 Flying Bomb kills.

The increased cooling requirements of the Griffon engine meant that all radiators were much bigger and the underwing housings were deeper than previous versions.

[22] I remember being greatly delighted with it; it seemed to me that from this relatively simple conversion, carried out with a minimum of fuss and bother, had come up with something quite outstanding ...

It fully justified the faith of those who, from the early days in 1939, had been convinced that the Griffon engine would eventually see the Spitfire into a new lease of life ...

Wing Commander Peter Brothers, O/C Culmhead Wing in 1944–1945 and a Battle of Britain veteran; It was truly an impressive machine, being able to climb almost vertically – it gave many Luftwaffe pilots the shock of their lives when, having thought they had bounced you from a superior height, they were astonished to find the Mk XIV climbing up to tackle them head-on, throttle wide open!

It was identical in most respects including engine (the Griffon 65) and cockpit enhancements, but it carried extra fuel and had a revised, stronger wing structure.

In 1951, Hainan Island (People's Republic of China) was targeted at the behest of US Naval Intelligence for RAF overflights, using Spitfire PR Mk 19s based at Kai Tak Airport in Hong Kong.

This information was needed in case RAF Lightnings might have to engage North American P-51 Mustangs in the Indonesian conflict of the time.

The proposed new design was called the Mk 21, which at first displayed poor flight qualities that damaged the excellent Spitfire reputation.

LA201's poor flight control qualities during trials in late 1944 and early 1945, led to a damning report from the Air Fighting Development Unit ...it must be emphasised that although the Spitfire 21 is not a dangerous aircraft to fly, pilots must be warned ... in its present state it is not likely to prove a satisfactory fighter.

Jeffrey Quill commented that, The AFDU were quite right to criticise the handling of the Mark 21 ... Where they went terribly wrong was to recommend that all further development of the Spitfire family should cease.

The squadron had little opportunity to engage the Luftwaffe before the war ended but scored a rare success on 26 April 1945, when two Spitfire Mk 21s shot up and claimed to have sunk a German midget submarine which they caught on the surface.

It was hoped that this would improve the pilot's view over the nose in flight and increase the high speed and dive performance of the aircraft.

All had the larger "Spiteful" tail units; modifications were also made to the trim tab gearings to perfect the F Mk 24's handling.

The F Mk 24 achieved a maximum speed of 454 mph (731 km/h) and could reach an altitude of 30,000 ft (9,100 m) in eight minutes, putting it on a par with the most advanced piston-engined fighters of the era.

As a fighter, the F Mk 24 armament consisted of four 20 mm Hispano cannon – operational experience had proved that the hitting power of these larger weapons was necessary to overcome the thicker armour encountered on enemy aircraft as the war progressed.

These remarkable increases in performance arose chiefly from the introduction of the Rolls-Royce Griffon engine in place of the famous Merlin of earlier variants.

Rated at 2,050 hp (1,530 kW), the 12-cylinder Vee liquid-cooled Griffon 61 engine had a two-stage supercharger, giving the Spitfire the exceptional performance at high altitude that had been sometimes lacking in early models.