The Supermarine Spitfire was a British single-seat fighter aircraft that was used by the Royal Air Force and many other Allied countries throughout the Second World War. The Spitfire continued to be used as a front line fighter and in secondary roles into the 1950s. It was produced in greater numbers than any other British aircraft and was the only British fighter in continuous production throughout the war.

The Spitfire was designed as a short-range, high-performance interceptor aircraft by R. J. Mitchell, chief designer at Supermarine Aviation Works (which operated as a subsidiary of Vickers-Armstrong since 1928). Mitchell continued to refine the design until his death from cancer in 1937, whereupon his colleague Joseph Smith became chief designer. The Spitfire's elliptical wing had a thin cross-section, allowing a higher top speed than several contemporary fighters, including the Hawker Hurricane. Speed was seen as essential to carry out the mission of home defence against enemy bombers.]

During the Battle of Britain (July–October 1940), the Spitfire was perceived by the public as the RAF fighter of the battle, though the more numerous Hawker Hurricane shouldered a greater proportion of the burden against the Luftwaffe. The Spitfire units had a lower attrition rate and a higher victory-to-loss ratio than those flying Hurricanes.

After the Battle of Britain, the Spitfire became the backbone of RAF Fighter Command, and saw action in the European, Mediterranean, Pacific and the South-East Asian theatres. Much loved by its pilots, the Spitfire served in several roles, including interceptor, photo-reconnaissance, fighter-bomber, carrier-based fighter, and trainer. It was built in many variants, using several wing configurations. Although the original airframe was designed to be powered by a Rolls-Royce Merlin engine producing 1,030 hp (768 kW), it was adaptable enough to use increasingly powerful Merlin and later Rolls-Royce Griffon engines; the latter was eventually able to produce 2,035 hp (1,520 kW).

Mk IX (type 361)
In the early months of 1942, with the clear superiority of the Focke Wulf Fw 190 over the Spitfire VB, there was much pressure to get Spitfires into production using the new two-stage supercharged Merlin 61 engine. In September 1941 the Spitfire Mk III prototype N3297 had been converted by Rolls-Royce at their Hucknall plant to take a Merlin 60, which had been specifically designed for use in the Wellington Mk VI high altitude bomber.

The performance increase was described as a "quantum leap" over that of the Mk VB and another Spitfire airframe, R6700 was modified to take the new engine. Although design work on the Mk VII and VIII series was under way, these would take over a year to get into production and a counter to the Fw 190 was urgently needed. The Air Ministry made the decision that Mk VC airframes should be converted to take the more powerful engine and, as a result, many of the early IXs were converted Mk VCs which did not have any of the refinements which later appeared. These could be identified by the Type C wing with the large double blisters over the inner cannon bays and the identification light on the fuselage spine, behind the aerial mast. The elevator horns were also smaller in size than that of most Mk IXs which had larger horn balances. The first IX was a modified VC AB505, which was tested by the Air Fighting Development Unit (AFDU) in April 1942.

The performance of the Spitfire IX is outstandingly better than the Spitfire V especially at heights above 20,000 feet. On the level the Spitfire is considerably faster and climb is exceptionally good. It will climb easily to 38,000 feet and when levelled off there can be made to climb in stages to above 40,000 feet by building up speed on the level and a slight zoom. Its manoeuvrability is as good as a Spitfire V up to 30,000 feet and above and is very much better. At 38,000 feet it is capable of a true speed of 368mph and is still able to manoeuvre well for fighting.

Although the Mk IX's airframe did not have the aerodynamic and strength improvements, or the modified control surfaces of the Mk VII and VIII, the Mk IX still proved to be an effective counter to the Focke Wulf Fw 190.

In September 1942, the "High Altitude Flight" was set up at RAF Northolt to counter the threat of the high altitude Junkers Ju 86 R bombers. Two Spitfire IXs converted from VCs were stripped of armour, the .303 Brownings and other equipment and repainted in a lightweight PRU blue finish. One of them successfully intercepted a Ju 86R at over 40,000 feet (12,000 m).

Production of the Mk IX finished at Supermarine in June 1943, with production continuing exclusively at the Castle Bromwich factory. Several major and large numbers of minor improvements were progressively introduced to Mk IXs, some of which were used on other Spitfire variants.

The Merlin 61 was phased out early in 1943 in favour of the Merlin 63 and 63A. The new engine had increased power resulting from engine improvements and engine limitations of +18 lbs./sq.in and 3,000 rpm (5 minute combat). During the second half of 1943, production of the Merlin 63 powered F Mk IX was discontinued in favour of the Merlin 66 powered LF Mk IX. Early production Spitfire IXs suffered from vapour locks in the fuel lines resulting from fuel evaporating if the aircraft was parked in direct sunlight. As a result of this the gun-camera was moved from the port wingroot to the starboard wingroot and a fuel cooler, fed by a small round air-intake was fitted in its place. This fuel cooler was also fitted to early PR Mk XIs

Production of the LF Mk IXs, frequently referred to as the Spitfire IXB, initially ran in parallel with the Merlin 63 powered Marks. This version first became operational in March 1943 with the Biggin Hill Wing, comprised at the time of 611 and 341(Free French) Squadrons. This type was by far the most produced of the Spitfire IX variants, with over 4000 built. The maximum power of the Merlin 66 was 1,720 hp (1,280 kW) at 5,750 ft (1,283 kW at 1,752 m) and the maximum speed of the Spitfire LF IX was 404 mph (650 km/h) at 21,000 feet (6,400 m). The Merlin 66 introduced a new Bendix-Stromberg injection carburettor, which replaced the earlier S.U float carburettor.

The H.F IX was powered by the specialised high altitude Merlin 70 and entered service in the Spring of 1944. Serial listings show that the HF Mk IX was produced in relatively limited numbers when they were required, with priority being given to versions rated for low and medium altitudes Maximum power of the Merlin 70 was 1,710 hp (1,280 kW) at 11,000 ft (1,275 kW at 3,353 m): maximum speed of the Spitfire HF.IX was 405 mph (652 km/h) at 25,400 feet (7,700 m) at an all-up weight of 7,320 lbs (3,320 kg).

Also introduced in early 1944 was a new Mark II Gyro gunsight. This gunsight calculated the correct angle of deflection to use when leading the target. Its introduction doubled the effectiveness of RAF gunnery and was a major factor in Allied air superiority.

The capacity of the main fuel tanks was 48 gal for the upper tank and 37 gal for the lower, for a total internal capacity of 85 gal. Jettisonable "slipper tanks" of 30, 45 or 90 gal could be carried under the centre-section. As an alternative a cylindrical 50 gal drop tank, adapted from those carried by long range Hawker Typhoons, could be carried on the fuselage bomb rack used on most Mk IXs of the Second Tactical Air Force. To further increase the combat radius some late production Mk IXs were fitted with additional internal self-sealing fuel tanks in the rear fuselage: the upper tank carried 41 gal and the lower 34 gal. When both were full this enabled a ferry range of over 1,200 miles (1,900 km), although they made the aircraft unstable in flight and only straight flight and gentle manoeuvres at low altitudes were recommended by the pilot's manual. The pilot was also warned to avoid instrument flying whenever possible.

Late production Mk IXs, in common with Mk XVIs introduced a new upper engine cowling which incorporated a distinct bulged top surface; this design was adopted for the Mk XVI to allow for the modified intercooler of the Packard Merlin 266, which had an integral header tank.

The cut-down rear fuselage and bubble canopy, trialled on a Spitfire Mk VIII, was incorporated into very late production Mk IXs. With the cut down fuselage the lower forward fuselage tank capacity was increased to 47 gallons, while the rear fuel tanks were decreased to a capacity of 66 gallons. These were the rarest of the Mk IXs and many of them featured the "clipped" wings. The great majority of these saw postwar service with the SAAF, both in South Africa and in deployment in Korea during the 1950s.

During early 1945, some Spitfire IXEs and XVIEs of 74 Squadron were fitted with a single RP-3 rocket under each wing. This was believed to be the only RAF Spitfire unit to use rockets operationally during the Second World War.

Pending development of a dedicated Merlin 61 powered PR Spitfire (the Mk XI) at least three Mk IXs (BS338, BS339 and BS473) were taken off the production line and modified to carry two vertical cameras in the rear fuselage. The first of these was delivered to 541 Squadron at RAF Benson on 30 November 1942. Subsequently another 15 Mk IXs were converted to PR Mk IXs (factory designation type 374) at the Vickers-Armstrong Worthy Down facility. As well as incorporating camera equipment, a wrap-around PR type windscreen was fitted and a larger oil tank was installed under the nose. All armament was removed and a PRU Blue finish applied. These aircraft lacked the "wet wing" tanks, meaning that the PR Mk IX relied on drop tanks for extra range. The most famous PR Mk IX missions involved carrying out reconnaissance missions in preparation for Operation Chastise, the 617 "Dambusters" Squadron attacks on the Ruhr dams. A PR Mk IX photographed the dams the day before the operation and photographed the Moehne Dam after the operation.

FR IXs were standard, armed Mk IXs modified with a single, port-facing, oblique camera. These aircraft were used for low altitude "Dicing" missions in tactical support of army operations. 16 Squadron, which was a unit of the 2nd TAF, used several FR Mk IXs (painted a pale, "Camoutint" Pink, which provided excellent camouflage under cloud cover) to photograph the Arnhem area before and during Operation Market Garden.Another unit using FR Mk IXs was 318(Polish) "City of Gdańsk" Fighter-Reconnaissance Squadron based in the Mediterranean.

In total, 5656 Mk IXs were built, 561 by Supermarine and 5,095 by Castle Bromwich.[20] Production of the Mk IX ended in April 1945 and, in combination with the Mk XVI this "compromise" was produced in larger numbers than any other Spitfire variant.

1943 Supermarine Spitfire Mk IX - Specifications
Length: 31 ft. 3½ in.
Wingspan: Standard (F) 36 ft. 10 in.
Clipped (LF) 32 ft. 2 in.
Height: 11 ft. 9 in.
Wing Area: 242 sq. ft. (F) or 231 sq. ft. (LF)
Empty Weight: 5,610 lbs. (2,545 kg)
Max. Takeoff Weight: 7,500 lbs. (3,402 kg)
Engine: One V-12 Rolls-Royce Merlin 61, 1,515 hp, or Merlin 63, 1,712 hp
Maximum Speed: 408 mph (657 kph) at 25,000 ft.
Service Ceiling: 4,000 ft.
Range w/o drop tank: 434 miles (698 km)
Armament: Two Hispano 20mm cannon and either four .303 machine guns or two .50 machine guns; 1,000 lbs of bombs (500 centerline, 250 each wing)
Number built: Mk IX 5,665
All Variants: 20,334