Early career and promotion In 1916, Mitchell joined the
Supermarine Aviation Works at
Southampton, possibly for a
probationary period. Since its formation in 1912, the company had specialised in building
flying boats, producing its first aircraft, the
Pemberton-Billing P.B.1, in 1914. During the First World War, Supermarine was taken over by the British Government, and during this period the company produced the first British single-seat flying boat
fighter, the
Supermarine Baby. moored at the start of the 1919
Schneider Trophy race. The 25-year-old Mitchell is likely to have played a role in the development of the aircraft. On joining the company, Mitchell was given the opportunity to develop skills in a number of roles, so as to gain experience of the aircraft industry. His basic engineering training would have helped him to become established, as he adjusted from working with locomotives to understanding aeroplanes. A competent mathematician, Mitchell's ability to think creatively and use his intuition when looking at a design was soon recognised. The earliest record of his work at Supermarine is as a
draughtsman, and dates from 1916. By 1917, he had become assistant to the company's owner and designer,
Hubert Scott-Paine. He is likely to have played a role in the development of the Baby when in 1919 it was adapted for racing for the
Schneider Trophy, and was renamed the
Supermarine Sea Lion. In 1918, Mitchell was promoted to become the works manager's assistant. When Supermarine's chief designer William Hargreaves left the company in the summer of 1919, he was replaced by Mitchell, who took up his new duties later that year, leading a team that had in 1918 consisted of six draughtsmen and a secretary. Following his promotion, the 19-year-old returned to Staffordshire and married his fiancée Florence Dayson, an infant school headmistress, who was 11 years his senior. By 1921 he had become Supermarine's chief engineer. Following the departure of Scott-Paine in November 1923, Mitchell was able to negotiate a new contract, which led to greater influence in the company. The 10-year contract was a sign of his indispensability to Supermarine. It is unclear how it was that Mitchell was so quickly promoted when he was still a young man, as few documents relating to his early career have survived. However, his early promotion was not unusual at that time; other men of Mitchell's age held similar positions in other aircraft companies. Decades after his death, when approached for information about him, those surviving Supermarine colleagues who had known Mitchell were reluctant to divluge their personal memories.
1920s civilian and military aircraft designs hull and
Napier Lion engine prior to be installed. Mitchell is standing second-to-left. Between 1920 and 1936, Mitchell designed 24 aeroplanes. His early projects often involved adapting Supermarine's earlier aircraft; in June 1920 the Air Ministry announced a civilian aircraft competition, and Supermarine's entry for the competition was the
Commercial Amphibian, an adaptation by Mitchell of the company's
Supermarine Channel. The Amphibian finished second, but was judged the best of the three entrants in terms of design and reliability. His redesigned Supermarine Baby, renamed the
Supermarine Sea King, was exhibited the
Olympia International Aero Exhibition in 1920, the first international exhibition to be held in the UK since the end of World War I. In 1922, the Chilean government bought a Channel, modified by Mitchell. That year he redesigned a version of the Commercial Amphibian, the
Supermarine Sea Eagle. Mitchell produced new designs for aircraft early in his career; he designed the
Supermarine Seal II in 1920, and the
triplane Flying Boat Torpedo Carrier the following year. The historian Ralph Pegram notes that the unbuilt Torpedo Carrier reveals the "first true indication of Mitchell's thoughts as a designer". In 1921 work began on the
Supermarine Swan, a commercial carrier, but only the prototype was built. The
Supermarine Seagull II—later used as the basis for future designs—began to receive production orders in 1922. The Amphibian Service Bomber was designed by Mitchell in 1924. Renamed the
Supermarine Scarab, 12 aircraft were bought by the
Spanish Navy; they remained in service until 1928. , one of the most successful flying boats of the between-war period Supermarine's first design for a land aircraft, the
Supermarine Sparrow, competed unsuccessfully during the Air Ministry's Light Air Competition of 1924, and subsequently failed to gain orders. A variant, the Supermarine Sparrow II, was used by Mitchell to test his different
airfoil designs. Work on the
Supermarine Southampton started in March 1924. It flew for the first time the following March, and entered service in July 1925. By the end of 1925, Mitchell's team had designed the Southampton II—the Southampton but with a metal hull. The plane, more powerful, lighter, and more durable than its predecessor, flew for the first time in 1927. A
paper by Mitchell on the use of the Southampton appeared in the March 1926 edition of
Flight magazine. In 1928, a flight of Supermarine Southampton IIs left
Felixstowe on 14 October for Australia, and returned to the UK on 11 December. The expedition provided Mitchell's design team with valuable information about operating aircraft in the
tropics. The Southampton was one of the most successful flying boats of the between-war period, and established Britain as a leading developer of maritime aircraft. It was used to equip six RAF
squadrons up to 1936. In 1926, the Air Ministry issued specification 21/26 as a way to address the need for new fighter aircraft, and Mitchell's design team, which he had re-organised that year into separate drawing and technical offices, responded with a number of designs, including the Single Seat Fighter. By this time, Supermarine was moving away from wooden
amphibious aircraft. The company concentrated instead on designing larger metal flying boats, such as the 3-Engined Biplane Flying Boat, designed in November 1927. The
Supermarine Air Yacht, and a new design, the Southampton X (not related to other planes with the same name), was ordered in June 1928. Mitchell dispensed with the complicated curved surfaces for the wings and the hulls of the Air Yacht and the Southampton X, and as a result these aircraft appeared "boxy". Specification R.6/28, issued in 1928, resulted in a series of designs by Supermarine for a six-engined flying boat, with one of designs being a radical departure for Mitchell—it had a newly designed
cantilever wing with a large surface area and cross section. The aircraft was never built. From 1929 to 1931, he continued to design aircraft based on the Southampton and the Southampton X, such as the Supermarine Sea Hawk and its variant the Sea Hawk II, the
Type 179, the
Nanok and the
Seamew.
New designs, production orders and patents (1929–1934) GB 329411 A In February 1929, Mitchell submitted
patent GB 329411 A, "Improvements in the Cooling System of Engines for Automotive Vehicles", a
condenser to be placed within the wings of an aircraft. The
Air Ministry rejected Supermarine's proposal for such a wing-cooled aircraft, but, in May 1929 a new specification allowed Mitchell to use his ideas again. A similar patent was submitted in 1931. The condenser was used in the
Type 232, produced in April 1934, which was never put up for
tender. During the early 1930s, many of Mitchell's ideas never went past the early design stages. Attempts by the company to sell a 5-engined flying boat failed when a contract was cancelled in early 1932, leading to job losses and wage cuts at Supermarine. However, in 1933, the company's fortunes were revived when it received an order for 12 Scapas (previously the Southampton IV) under the specification R.19/33, the first contract for a new design by Mitchell since 1924. This order was followed by orders for the
Supermarine Stranraer, which went into production in 1937. After the first Seagull V flew in June 1933, the
Royal Australian Air Force showed an interest, and 24 planes were ordered. The same year, the RAF placed an initial order of 12 aircraft, now renamed the
Supermarine Walrus. Following the issuance of Air Ministry specification 5/36, Mitchell worked on a redesigned version of the Walrus, which was given the name
Sea Otter. Work on the Sea Otter was completed after Mitchell's death in 1937, and it first flew in September 1938. In October 1934, Mitchell published an article in the
Daily Mirror, 'What is happening now in Air Transport?', in which he predicted that air transport would prove to be the safest form of transport.
Schneider trophy races (1922–1931) Mitchell and his design team worked on a series of racing seaplanes, built to compete in the Schneider Trophy competition. His team included Alan Clifton (later head of the Technical Office), Arthur Shirvall, and
Joseph Smith. These men were fundamental to Supermarine's success, as was the
National Physical Laboratory (NPL), which provided invaluable support, guidance and scientific expertise in the form of detailed reports. The competition helped to place Mitchell at the forefront of aviation design.
Sea Lion series (early 1920s) Mitchell developed the
Supermarine Sea King II to become the
Sea Lion II, which competed for the 1922 Schneider Trophy in
Naples. The Sea Lion II won the race, flying at an average speed of . There was not enough time for Supermarine to design a new flying boat for the 1923 competition, so the Sea Lion II was borrowed back from the Air Ministry to allow Mitchell to adapt it. He increased its maximum speed by , achieved with the assistance of
D. Napier & Son, who supplied the
Lion III engine. To reduce the effects of
drag forces, Mitchell reduced the
wingspan from , modified the
struts,
floats and hull, and changed the way the engines were fitted. For the 1923 contest, two of the three British entrants were irreparably damaged before the race, leaving the Sea Lion III to compete alone. The United States team, flying
Curtiss seaplanes, dominated the competition, with the winning pilot,
David Rittenhouse, managing to reach a top speed of .
Supermarine S.4 (1925) , which crashed and sank during navigation trials in 1925 Even whilst the Sea Lion II was being modified at the
Woolston works, Mitchell was working on a new plane, as Supermarine knew the American monoplane was the best design then available. The
Supermarine S.4—the name was designated by Mitchell, with "S" standing for
Schneider—was a joint Napier/Supermarine venture. The Supermarine team was backed by the Air Ministry, and had greater freedom than was given by the US government to their designers. The S.4 was described after Mitchell's death as "his first outstanding success". The S.4 lacked the newly designed surface radiators, at that time still unavailable, but it was aerodynamic and
aesthetically pleasing. Trial speeds reached and created a sensation in the
press. The S.4 crashed before the 1925 race, for reasons that were never clearly established. On the day of the navigation trials it stalled before falling flat into the sea from . When the pilot Henri Biard was rescued by a
launch, Mitchell, who was on board the rescue launch, jokingly asked the injured man: "Is the water warm?"
1926 and 1927 competitions , winner of the
Schneider Trophy at
Venice in 1927 The Air Ministry, the
Society of British Aircraft Constructors and the
Royal Aeronautical Society (RAeS) decided against challenging for the Schneider Trophy in 1926, but Mitchell was able to confirm that Supermarine would be ready for the race. His work at the NPL started in November that year. From wind tunnel tests at the NPL he learned that the S.4's
radiators had created a third of the aircraft's total drag, and without this it would have been the most streamlined aircraft in the world. British aircraft companies intended to produce entries for the 1926 race, but the nature of the specifications issued by the Air Ministry meant that no aircraft could be completed and tested in time to be entered. Two Supermarine S.5 seaplanes were entered for the 1927 contest, which was held in
Venice. Mitchell understood that a monoplane on twin floats produced lower drag than any other aircraft type of its day, and was convinced by
wind tunnel tests at the NPL that the cantilever wing design was too heavy and should be abandoned. The NPL had demonstrated that flat-surfaced skin radiators reduced drag better than the corrugated variety preferred by American designers, so Mitchell used them to improve the S.5. He reduced the fuselage cross section area so that it was 35 per cent less than the area of the S.4—and complained about the RAF's pilots being too large to fit into the resulting S.5's cockpit. The fuselage skin thickness was decreased by using
duralumin. Witnessed by the Italian dictator
Benito Mussolini, along with a huge crowd gathered on the
Venice Lido, the two Supermarine S.5s were the only seaplanes to finish the race, coming first and second. The third British entrant, a
Gloster IV, along with the three Italian competitors flying
Macchi M.52s, were forced to drop out of the race. Mitchell had been elected to the RAeS in 1918. In 1927 he was awarded the society's silver medal.
Supermarine S.6B (1931) Britain's final entry in the series, the
Supermarine S.6B, marked the culmination of Mitchell's quest to "perfect the design of the racing seaplane". It was sponsored by a wealthy
philanthropist,
Lady Houston, who donated after the British Government decided not to enter an
RAF team for the 1931 contest. Mitchell opted to design an improved version of the S.6, whilst making as few changes as possible. The improvements that were made included a more powerful engine, and provision was made for such effects as the increase in engine-produced heat and extra torque, and the greater quantities of cooling oil and fuel required. The S.6B was a larger seaplane than the S.6, and had to be given a more efficient cooling system, and a stronger frame. The S.6B competed the course successfully, and won the 1931 race. As the Schneider Trophy rules included the stipulation that the contest would end when any one country managed to win the trophy three times in five years, the S.6B's victory won the contest outright for Britain. The aircraft went on to break the world air speed record when it reached a speed of that year. Mitchell was awarded the
Commander of the Most Excellent Order of the British Empire (CBE) on 29 December 1931 for services in connection with the Schneider Trophy contest.
Type 224 In 1930,
specification F7/30 was issued for a fighter aircraft able to be used by both day and night squadrons. Mitchell's proposed design, the
Type 224, was one of three monoplane designs made into prototypes for the Air Ministry. The final design incorporated an open cockpit, four
Vickers machine guns, and a
Rolls-Royce Goshawk engine, along with a fixed undercarriage. Also included was an inverted gull wing, needed due to the demands of the engine's cooling system. The wing lacked flaps, a requirement for the aircraft to land at safe speeds. Unofficially named the Spitfire, the Type 224 first flew in February 1934. The aircraft looked clumsy, and was inefficient, in part because the cooling system failed to prevent the engine from overheating. The RAF decided that the Type 224's performance was unsatisfactory, and selected the
Gloster Gladiator in preference.
Supermarine Spitfire in 1936 Whilst the Type 224 was still being built in 1933, Mitchell was proceeding with the design of the Type 300. This was to become his masterpiece, the
Supermarine Spitfire. He cleaned up the design of the Type 224, using the same engine but incorporating a shorter wing and a retractable undercarriage. The Air Ministry rejected Mitchell's design, but he modified it, for instance by making the wing thinner and shorter, by including the newly designed
Rolls-Royce Merlin engine, and by making use of an innovative new cooling system—the latter being an example of his willingness to accept ideas from other people. For a short period, design work continued using private funding, but in December 1934 the Air Ministry contracted Supermarine to construct a prototype that was based on Mitchell's design. Mitchell objected to the Air Ministry's insistence that the Spitfire be modified to have a
tail wheel. At the time he was not told that, in preparation for a future war, the government had decided to build
hard surface runways for the RAF, a decision that meant the modification to the Spitfire was necessary. The prototype, given the
serial K5054, first flew on 6 March 1936, at
Eastleigh,
Hampshire. Mitchell witnessed the flight. Despite being ill, he travelled to Eastleigh during the flight tests for
K5054. In June 1936, before the prototype had completed being trialled, the Air Ministry placed an order for 310 Spitfires. Many of the technical advances in the Spitfire were made by people other than Mitchell: the thin elliptical wings were designed by the Canadian
aerodynamicist Beverley Shenstone, and the Spitfire shared similarities with the
Heinkel He 70 Blitz. The under-wing radiators had been designed by the
Royal Aircraft Establishment, and
monocoque construction had been first developed in the United States. Mitchell's achievement lay in the merger of these different influences into a single design, originating from his "unparalleled expertise in high-speed flight... and a brilliant practical engineering ability, exemplified in this instance by the incorporation of vital lessons learned from Supermarine's unsuccessful type 224 fighter". The quality of the design enabled the Spitfire to be continually improved throughout World War II. ==Illness and final years==