In 1931, the Air Ministry released specification F7/30, calling for a modern fighter capable of a flying speed of 250 mph (400 km/h) to replace the Gloster Gauntlet biplane. R. J. Mitchell designed the Supermarine Type 224 to fill this role in competition with the Blackburn F.3 and Westland F.7/30 and privately funded designs from Gloster. The 224 was an open-cockpit monoplane with bulky gull wings and a large, fixed, spatted undercarriage powered by the 600-horsepower (450 kW), evaporatively cooled Rolls-Royce Goshawk engine.^[5] It made its first flight in February 1934.^[6] Of the seven designs tendered to F7/30, the Gloster Gladiator biplane was accepted for service.^[7]

The Type 224 was a big disappointment to Mitchell and his design team, who immediately embarked on a series of "cleaned-up" designs, using their experience with the Schneider Trophy seaplanes as a starting point.^[7] This led to the Type 300, with retractable undercarriage and a wingspan reduced by 6 ft (1.8 m). This design was submitted to the Air Ministry in July 1934, but was not accepted.^[8] It then went through a series of changes, including the incorporation of an enclosed cockpit, oxygen-breathing apparatus, smaller and thinner wings, and the newly developed, more powerful Rolls-Royce PV XII V-12 engine^{[nb 1]}, which was later named the "Merlin". In November 1934, Mitchell, with the backing of Supermarine's owner Vickers-Armstrong, started detailed design work on this refined version of the Type 300.^[9]

On 1 December 1934, the Air Ministry issued contract AM 361140/34, providing £10,000 for the construction of Mitchell's improved Type 300 design.^[10] In April 1935 Ralph Sorley spoke to Mitchell about the new specification F10/35 which called for armament of at least six and preferably eight guns while at the same time removing bomb carry requirement and reducing fuel capacity. Mitchell foresaw no problem adding the guns and welcomed the reduction which would reduce weight. A specification for an eight-gun fighter, F5/34 had come from a recommendation by Squadron Leader Ralph Sorley of the Operational Requirements section at the Air Ministry.^[11][12] In the redesign the change was made from Vickers machine guns to .303 in (7.7 mm) Brownings),^[13] and the fuel tankage dropped to 75 gallons from 94.^[14]

On 5 March 1936,^{[15][nb 2]} the prototype (K5054), fitted with a fine-pitch propeller to give more power for take-off, took off on its first flight from Eastleigh Aerodrome. At the controls was Captain Joseph "Mutt" Summers, chief test pilot for Vickers, who is quoted as having said, "don't touch anything" on landing.^{[16][nb 3]} This eight-minute flight^[11] came four months after the maiden flight of the contemporary Hurricane.^[18]

K5054 was fitted with a new propeller, and Summers flew the aircraft on 10 March 1936; during this flight, the undercarriage was retracted for the first time.^[19] After the fourth flight, a new engine was fitted, and Summers left the test flying to his assistants, Jeffrey Quill and George Pickering. They soon discovered that the Spitfire^{[nb 4][22]} was a very capable aircraft, but not perfect. The rudder was oversensitive, and the top speed was just 330 mph (528 km/h), little faster than Sydney Camm's new Merlin-powered Hurricane.^[24] A new and better-shaped, two-bladed, wooden propeller allowed the Spitfire to reach 348 mph (557 km/h) in level flight in mid-May, when Summers flew K5054 to RAF Martlesham Heath and handed the aircraft over to Squadron Leader Anderson of the Aeroplane & Armament Experimental Establishment (A&AEE). Here, Flight Lieutenant Humphrey Edwardes-Jones took over the prototype for the RAF.^[25] He had been given orders to fly the aircraft and then to make his report to the Air Ministry on landing. Edwardes-Jones' report was positive; his only request was that the Spitfire be equipped with an undercarriage position indicator.^[26] A week later, on 3 June 1936, the Air Ministry placed an order for 310 Spitfires,^[27] at a cost of £1,395,000.^[28] before the A&AEE had issued any formal report. Interim reports were later issued on a piecemeal basis.^[29]

The British public first saw the Spitfire at the RAF Hendon air display on Saturday 27 June 1936. Although full-scale production was supposed to begin immediately, numerous problems could not be overcome for some time, and the first production Spitfire, K9787, did not roll off the Woolston, Southampton assembly line until mid-1938.^[2]

In February 1936, the director of Vickers-Armstrong's, Sir Robert MacLean guaranteed production of five aircraft a week, beginning 15 months after an order was placed. On 3 June 1936, the Air Ministry placed an order for 310 aircraft.^[28] Full-scale production of the Spitfire began at Supermarine's facility in Woolston, but the order clearly could not be completed in the 15 months promised. Supermarine was a small company, already busy building Walrus and Stranraer flying boats, and Vickers was busy building Wellington bombers. The initial solution was to subcontract the work.^[28] Although outside contractors were supposed to be involved in manufacturing many important Spitfire components, especially the wings, Vickers-Armstrong's (the parent company) was reluctant to see the Spitfire being manufactured by outside concerns, and was slow to release the necessary blueprints and subcomponents.^[30]

As a result of the delays in getting the Spitfire into full production, the Air Ministry put forward a plan that its production be stopped after the initial order for 310, after which Supermarine would build Bristol Beaufighters. The managements of Supermarine and Vickers were able to convince the Air Ministry that production problems could be overcome, and a further order was placed for 200 Spitfires on 24 March 1938. The two orders covered the K, L, and N prefix serial numbers.^[30]

The first production Spitfire came off the assembly line in mid-1938^[2] and was flown by Jeffrey Quill on 15 May 1938, almost 24 months after the initial order.^[31] The final cost of the first 310 aircraft, after delays and increased programme costs, came to £1,870,242 or £1,533 more per aircraft than originally estimated.^[32] A production aircraft cost about £9,500. The most expensive components were the hand-fabricated and finished fuselage at roughly £2,500, then the Rolls-Royce Merlin engine at £2,000, followed by the wings at £1,800 a pair, guns and undercarriage, both at £800 each, and the propeller at £350.^[33]

In 1935, the Air Ministry approached Morris Motors Limited to ask how quickly their Cowley plant could be turned to aircraft production. In 1936, this informal request for major manufacturing facilities was replaced by a formal scheme, known as the shadow factory plan, to boost British aircraft production capacity under the leadership of Herbert Austin. He was given the task of building nine new factories, and to supplement the British car-manufacturing industry by either adding to overall capacity or increasing the potential for reorganisation to produce aircraft and their engines.^[34]

In 1938, construction began on the Castle Bromwich Aircraft Factory (CBAF), next to the aerodrome, and the installation of the most modern machine tools then available began two months after work started on the site.^[32] Although Morris Motors, under Lord Nuffield, who was an expert in mass motor-vehicle construction, managed and equipped the factory, it was funded by the government. By the beginning of 1939, the factory's original estimated cost of £2,000,000 had more than doubled,^[35] and even as the first Spitfires were being built in June 1940, the factory was still incomplete, and suffering from personnel problems. The Spitfire's stressed-skin construction required precision engineering skills and techniques that were beyond the capabilities of the local labour force, and some time was required to retrain them. Difficulties arose with management, who ignored Supermarine's tooling and drawings in favour of their own, and the workforce continually threatened strikes or "slow downs" until their demands for higher wages were met.^[36]

In spite of promises that the factory would be producing 60 per week starting in April, by May 1940, Castle Bromwich had not yet built its first Spitfire.^[35][37] On 17 May, Minister of Aircraft Production Lord Beaverbrook telephoned Lord Nuffield and manoeuvred him into handing over control of the Castle Bromwich plant to his ministry.^[38] Beaverbrook immediately sent in experienced management staff and workers from Supermarine, and gave control of the factory to Vickers-Armstrong's. Although resolving the problems took time, in June 1940, 10 Mk IIs were built; 23 rolled out in July, 37 in August, and 56 in September.^[39] By the time production ended at Castle Bromwich in June 1945, a total of 12,129 Spitfires, comprising 921 Mk IIs,^[40] 4,489 Mk Vs, 5,665 Mk IXs,^[41] and 1,054 Mk XVIs had been built.^[40]

During the Battle of Britain, the Luftwaffe made concerted efforts to destroy the main manufacturing plants at Woolston and Itchen, near Southampton. The first bombing raid, which missed the factories, came on 23 August 1940. Over the next month, other raids were mounted, until, on 26 September 1940, both factories were destroyed,^[42] with 92 people killed and a large number injured. Most of the casualties were experienced aircraft-production workers.^[43]

Fortunately for the future of the Spitfire, many of the production jigs and machine tools had already been relocated by 20 September, and steps were being taken to disperse production to small facilities throughout the Southampton area.^[42] To this end, the British government requisitioned the likes of Vincent's Garage in Station Square, Reading, which later specialised in manufacturing Spitfire fuselages, and Anna Valley Motors, Salisbury,^[44] which was to become the sole producer of the wing leading-edge fuel tanks for photo-reconnaissance Spitfires.^[45]

A purpose-built works, specialising in manufacturing fuselages and installing engines, was built at Star Road, Caversham in Reading.^[43] The drawing office in which all Spitfire designs were drafted was moved to Hursley Park, near Winchester.^[46]

Four towns and their satellite airfields were chosen to be the focal points for these workshops:^[42] Southampton's Eastleigh Airport; Salisbury and the High Post and Chattis Hill^{[47][nb 6]} aerodromes; Trowbridge and RAF Keevil;^[48] and Reading's Henley and Aldermaston aerodromes. Completed Spitfires were delivered to the airfields on Commer "Queen Mary" low-loader trailers, there to be fully assembled, tested, then passed on to the RAF.^[43]

All production aircraft were flight tested before delivery. During the Second World War, Jeffrey Quill was Vickers Supermarine's chief test pilot, in charge of flight testing all aircraft types built by Vickers Supermarine. He oversaw a group of 10 to 12 pilots responsible for testing all developmental and production Spitfires built by the company in the Southampton area.^{[nb 7]} Quill devised the standard testing procedures, which with variations for specific aircraft designs operated from 1938.^[49][50] Alex Henshaw, chief test pilot at Castle Bromwich from 1940, was placed in charge of testing all Spitfires built at that factory. He co-ordinated a team of 25 pilots and assessed all Spitfire developments. Between 1940 and 1946, Henshaw flew a total of 2,360 Spitfires and Seafires, more than 10% of total production.^[51][52] Henshaw wrote that he would climb at full throttle at 2,850 rpm to the rated altitude, then dive at full power and 3,000 rpm, and trim the plane to fly by itself at 460 mph (740 km/h). A production test usually consisted of an initial circuit of under 10 minutes, and a main flight of 20 to 30 minutes. He commented that the more powerful later versions were heavier and did not handle as well as the earlier ones, for example in a flick-roll.^[53][54]

When the last Spitfire rolled out in February 1948,^[55] a total of 20,351 examples of all variants had been built, including two-seat trainers, with some Spitfires remaining in service well into the 1950s.^[1] The Spitfire was the only British fighter aircraft to be in continuous production before, during, and after the Second World War.^[56]

In the mid-1930s, aviation design teams worldwide began developing a new generation of fighter aircraft. The French Dewoitine D.520^[57] and the German Messerschmitt Bf 109, for example, were designed to take advantage of new techniques of monocoque construction, and the availability of new, high-powered, liquid-cooled, in-line aero engines. They also had refinements such as retractable undercarriages, fully enclosed cockpits, and low-drag, all-metal wings. These advances had been introduced on civil airliners years before, but were slow to be adopted by the military, who favoured the biplane's simplicity and manoeuvrability.^[58]

Mitchell's design aims were to create a well-balanced, high-performance fighter aircraft capable of fully exploiting the power of the Merlin engine, while being relatively easy to fly.^[59] At the time, with France as an ally, and Germany thought to be the most likely future opponent, no enemy fighters were expected to appear over Great Britain. German bombers would have to fly to the UK over the North Sea, and Germany did not have any single-engine fighters with the range to accompany them. To carry out the mission of home defence, the design was intended to allow the Spitfire to climb quickly to intercept enemy bombers.^[60]

The Spitfire's airframe was complex. The streamlined, semi-monocoque, duralumin-skinned fuselage had a number of compound curves built up over a skeleton of 19 formers, also known as frames. These started from frame number one, immediately behind the propeller unit, to the tail unit attachment frame. The first four frames supported the glycol header tank and engine cowlings. Frame five, to which the engine bearers were secured, supported the weight of the engine and its accessories. This was a strengthened double frame which also incorporated the fireproof bulkhead, and in later versions of the Spitfire, the oil tank. This frame also tied the four main fuselage longerons to the rest of the airframe.^[61] Behind the bulkhead were five U-shaped half-frames which accommodated the fuel tanks and cockpit. The rear fuselage started at the 11th frame, to which the pilot's seat and (later) armour plating were attached, and ended at the 19th, which was mounted at a slight forward angle just forward of the fin. Each of these nine frames was oval, reducing in size towards the tail, and incorporated several lightening holes to reduce their weight as much as possible without weakening them. The U-shaped frame 20 was the last frame of the fuselage proper and the frame to which the tail unit was attached. Frames 21, 22 and 23 formed the fin; frame 22 incorporated the tailwheel opening and frame 23 was the rudder post. Before being attached to the main fuselage, the tail unit frames were held in a jig and the eight horizontal tail formers were riveted to them.^[62]

A combination of 14 longitudinal stringers and four main longerons attached to the frames helped form a light but rigid structure to which sheets of alclad stressed skinning were attached. The fuselage plating was 24, 20, and 18 gauge, decreasing in order of thickness towards the tail, while the fin structure was completed using short longerons from frames 20 to 23, before being covered in 22 gauge plating.^[63]

The skin of the fuselage, wings, and tailplane was secured by dome-headed rivets, and in critical areas such as the wing forward of the main spar where an uninterrupted airflow was required, with flush rivets. From February 1943 flush riveting was used on the fuselage on all Spitfire variants.^[64] In some areas, such as at the rear of the wing and the lower tailplane skins, the top was riveted and the bottom fixed by brass screws which tapped into strips of spruce bolted to the lower ribs. The removable wing tips were made up of duralumin-skinned spruce formers.^[65]

At first, the ailerons, elevators, and rudder were fabric-covered, but once combat experience showed that fabric-covered ailerons were impossible to use at high speeds a light alloy replaced the fabric, enhancing control throughout the speed range.^[66]

In 1934, Mitchell and the design staff decided to use a semi-elliptical wing shape to solve two conflicting requirements; the wing needed to be thin to avoid creating too much drag, but it had to be thick enough to house the retractable undercarriage, armament, and ammunition. An elliptical planform is the most efficient aerodynamic shape for an untwisted wing, leading to the lowest amount of induced drag. The ellipse was skewed so that the centre of pressure, which occurs at the quarter-chord position, aligned with the main spar, preventing the wings from twisting. Mitchell has sometimes been accused of copying the wing shape of the Günter brothers-designed Heinkel He 70,^[67] which first flew in 1932, but as Beverley Shenstone, the aerodynamicist on Mitchell's team, explained: "Our wing was much thinner and had quite a different section to that of the Heinkel. In any case, it would have been simply asking for trouble to have copied a wing shape from an aircraft designed for an entirely different purpose."^{[68][nb 8]}