The first production model of the Spitfire was the Mk I and it had a liquid-cooled, Merlin II engine capable of propelling the Mk I at speeds of up to 571.3 kmh. The armament of the Mk I consisted of eight .303 MGs, each with 300 rounds of ammunition. The Mk I proved to be quite a reliable fighter and was quickly a popular choice among British pilots. It also had higher climb rates and maneuvering than some of the early German fighters giving it the edge during key early confrontations. However, German pilots quickly learned that without engine injection, it was possible to cut power to the engine if one should force the spitfire to perform negative G forces. The maximum service ceiling of the aircraft was restricted to 9,700 meters, though the Spitfire would have difficulty at altitudes far lower.
The Spitfire had many variants throughout its service life. There were eventually 26 variants of Spitfire, not including the carrier based version, the Supermarine Seafire. The first variant was the Mk IA, [N 1] which was supplemented by the similar Mk II.[N 2] Some Mk IIA aircraft were fitted with a 40 UK gallon long range fuel tank under the outer starboard wing, and were known as the Type 343.
The next major variant was the Mk V (Type 349) with Merlin 45 engine. As well as A and B type wings, the Mk V introduced the 'universal' C type wing, which could carry four machine guns, one cannon and two machine guns or two cannons. Following introduction of the superior Fw 190, it was decided, as a matter or urgency, to fit a Mk V airframe with a series 60 Merlin engine, in order to produce a new variant known as the Mk IX (Type 361), which could serve as an interim type until the dedicated Mk VIII (Type 359) could be made readily available. Initially fitted with the 'C' wing, later Mk IX aircraft received the 'E' wing, which had a 0.5 in machine gun located in the inner cannon space, with the 20 mm cannon moved to the outer position.
The similar Mk XVI was fitted with Merlin engines built by Packard. Later Mk XVI aircraft had enlarged rudders and cut down fuselages with 'teardrop' rear vision hoods. Also produced alongside the standard Merlin engine variants, were the griffon engine variants designed to perform better than the standard Merlin engine variants. The first of these variants to go into service was the Mk XII which were most effective at low to mid altitudes. It was hoped that using clipped wings would perhaps improve the high altitude performance, to little result. However, these faults were eventually rectified. The first aircraft to do so was the Spitfire Mk XIV with a Griffon 65 engine using five propellers. After successfully completing its field test of intercepting V1 Rockets, the Mk XIV became operational by late 1944. The Mk 21 meanwhile was created with an enhanced airframe powerful enough to handle the power of the Griffon 61 engine, starting development in 1942. The new armament would be four 20mm Hispano Autocannons and no machine guns. However, the first Mk 21 aircraft had exceedingly poor flight characteristics. While through extensive trials in 1944 most of these issues were fixed, pilots still disliked the aircraft. It wasn't until January 1945 that the first Mk 21s were put into service. Similar to the Mk 21 was the Mk 22 with a new teardrop canopy and enlarged tail section.
During 1940-41 work on pressure cabins by the Royal Aircraft Establishment and Supermarine led to the development of a high altitude version of the Mk VB. Known as the Mk VI (Type 350), 100 examples of this version were built, and were equipped with a pressure cabin,[N 3] Merlin 47 engine,[N 4] four bladed propeller units with Jabo blades, and pointed wingtips, which increased the wingspan to 40 ft 2 in. Armament was similar to the VB. The Mk VI was followed by the Mk VII (Type 351), which was a Mk VI airframe redesigned to use the Merlin 60 series engine. in 1942, a pair of Mk VC in the Middle East were modified to undertake combat against high flying Ju-86 bomber and reconnaissance aircraft. One airframe eventually reached an altitude of almost 50,000 ft, despite neither aircraft being fitted with pressurization equipment.
Carrier Based versions
See entry for the Supermarine Seafire
The relative sophistication of the Spitfire led to a 1941 proposal for a two-seat version of the Spitfire, which could be used for advanced training purposes.[N 5] Although the idea was not officially adopted by the British until after the war, ES127, a Mk VC assigned to 261 Squadron at Catania in Sicily, was fitted with an open cockpit in front of the original. A Mk IX was modified in 1945 at No 1 Aircraft Depot, Leningrad for use as a two-seat trainer, by the installation of a second closed cockpit behind the regular one.
Shortly after D-Day, a Spitfire Mk Vc was modified under this designation to transport two barrels of beer across the English Channel. The 18 gallon barrels were provided by Sussex Brewers Henty and Constable, and carried under the wings by modified bomb racks, supplemented by a modified drop tank carried under the fuselage.
The idea of equipping Spitfire aircraft with floats was first considered in response to the Norwegian campaign of 1940. A single Spitfire Mk I was adapted to take floats similar to those fitted to the third Blackburn Roc, and was known as the Type 342,[N 6] but not completed before the end of the Norwegian campaign.
In 1942 the idea was revived, and Supermarine designed a set of floats with 90 percent buoyancy for the Spitfire Mk V. The floats were built and fitted by Folland Aircraft, and were attached to the wing spars some five feet from the centre-line using cantilever struts. The modified aircraft was tested as the Type 355, with one of the three examples built being flown in the Mediterranean area. in 1943 Folland fitted a set of floats to a Mk IX aircraft, which became the Type 385. All four aircraft had additional fin area, including an under fin to counteract destabilisation caused by the floats. 
The Air Ministry later requested submission of a proposal for a float variant of the F.21 Super Spitfire, but interest never progressed beyond the specification stage. Supermarine's involvement in Spitfire floatplanes was limited to design and specification work, with the actual conversions being carried out by Follands. 
In the late 1930s, as tension between Britain and Germany increased, the arms race in the air took on a literal form. At an international ﬂying meeting in Zurich in July 1937, the Germans boasted that a highly modiﬁed Me 109, winner of the Circuit des Alpes, was now the fastest ﬁghter in the world, capable of reaching 379 mph. The Air Ministry was unwilling to let this claim go unchallenged and Supermarine, working with Rolls-Royce, were instructed to build a special high-speed version of the Spitﬁre, which could break the world speed record for a land plane. The plane that the company came up with had a number of changes compared to the Mark I production Spitﬁre, including a reduced wing area, an absence of gun mountings and radio ﬁttings, a streamlined skid instead of a tailwheel, a new windscreen, a four-bladed propeller, and a highly polished ﬁnish. Above all, the aircraft had an adapted Merlin engine, based on the type used in the last Schneider Trophy winner and capable of 2,000 horsepower.
It was not until November I938 that the racing Spitfire was ﬁnally ready, by which time the Germans had extended their lead, as in June a Heinkel 100 had established a new record of 394 mph. However, Supermarine had not given up entirely and, in February 1939. the Speed Spitﬁre N.17 took off from Eastleigh and reached 408 mph. But more work was needed if a genuine challenge was to be mounted. This led to the radiator being replaced with a water tank and condenser fitted in early 1940, in place of the upper fuel tank. However, any lingering hopes were extinguished in March by the news from Germany that an He 100 had attained the breathtaking speed of 463 mph, smashing not only the world land plane speed record but also the absolute world speed record, until then held by the Italian Macchi MC 72 seaplane at 440 mph. There was no chance that even the most streamlined and altered Spitﬁre of 1939 could go at anything like that pace. [N 7]
Exchanging it's N.17 record attempt identity for it's original serial number of K9834, the Speed Spitfire was converted to 'standard' condition, with a normal windscreen, three bladed Rotol propeller, Merlin XII engine and conventional radiator and oil cooler. Delivered to the RAF's Photo Reconnaissance Unit in November 1940, the aircraft was soon fitted with an oblique F.24 camera, but as the upper fuel tank had not been restored, the aircraft's range was reduced, limiting it's usefulness in the reconnaissance role. After further use as a unit 'hack' and communication aircraft, as well as being flown over the Normandy beach head on D-Day by Air Commodore Bootham as his personal aircraft, K9834 was eventually struck of charge on 14 June 1946.
The Spitfire was introduced into RAF service on 11 August 1938, at RAF Duxford, Cambridgeshire, UK. They were used by 19 Sqn. It was designed by Reginald J. Mitchell, and was heavily based on his Schneider Trophy winning design, the S.6B. Unfortunately, Mitchell died before he had chance to see the Spitfire's success. The Spitfire was also used by many other allied countries throughout WWII, and it was continually used as a front line fighter up until the 1950s. The Spitfire was the most produced Allied fighter throughout WWII. It was most famous for its service defending against German aircraft during the Battle of Britain.
- Retrospective designation applied to distinguish the eight machine gun Type 300 aircraft from the IB version, the Type 331, which was fitted with two 20 mm cannon and four machine guns.
- The Mk II (Type 329) was basically the Mk I powered by a 1,175 hp Merlin XII. Built at Castle Bromwich, the Mk II incorporated all the improvements developed for the Mk I, such as constant speed airscrew, on the production line. Like the Mk I, the Mk II was built with A (eight machine gun) and B (two cannon and four machine gun) wings.
- This was fitted between the fore and aft cockpit bulkheads, and topped with a non sliding canopy to ease pressurisation. A Marshall blower provided an atmosphere differential of 2 lb per sq in, reducing apparent altitude from 40,000 ft to 28,000 ft.
- This was the high altitude version of the Merlin 45.
- A similar proposal for a two seat version of the Hawker Hurricane had been submitted in 1939.
- Also known as the Narvik Nightmare.
- Furthermore, the Heinkel record for a piston-engined plane was subsequently beaten when the Me 209 set a new record of 469 mph, This record, which was falsely attributed to a Bf 109R (A non existent sporting version of the Bf 109), stood for thirty years until it was beaten in 1969 by a Grumman Bearcat ﬂying at 482 mph.
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- McKinstry, Leo. Page 370
- Morgan, Eric B and Edward Shacklady. Spitfire - The History. 2000. ISBN 0 946219 48 6 Page 380
- Green, William. Famous Fighters of the Second World War. Purnell Book Services. 1975. Page 136
- Morgan, Eric B and Edward Shacklady. Page 386
- McKinstry, Leo. Portrait of a Legend - Spitfire. John Murray (Publishers). 2008. ISBN 0 7195 6875 6 Pages 128-129
- Caygill, Peter. Combat Legends Spitfire Mks I-V. Airlife Publishing. 2002. ISBN 1 84037 391 1 page 79
- Nowarra, Heinz. Aircraft & Legend Messerschmitt Bf 109, English Edition. Haynes Publishing Group (1989), ISBN 0 85429 729 4 Page 49
- Caygill, Peter - Spitfire page 77
- Morgan, Eric B and Edward Shacklady. Page 67
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