Due to a shortage of Brownings, which had been selected as the new standard rifle calibre machine gun for the RAF in 1934, early Spitfires were fitted with only four guns, with the other four fitted later.[93] Early tests showed that while the guns worked perfectly on the ground and at low altitudes, they tended to freeze at high altitude, especially the outer wing guns. This was because the RAF's Brownings had been modified to fire from an open bolt; while this prevented overheating of the cordite used in British ammunition, it allowed cold air to flow through the barrel unhindered.[94] Supermarine did not fix the problem until October 1938, when they added hot air ducts from the rear of the wing mounted radiators to the guns, and bulkheads around the gunbays to trap the hot air in the wing. Red fabric patches were doped over the gun ports to protect the guns from cold, dirt and moisture until they were fired.[95] Even if the eight Brownings worked perfectly, pilots soon discovered that they were not sufficient to destroy larger aircraft. Combat reports showed that an average of 4,500 rounds were needed to shoot down an enemy aircraft. In November 1938, tests against armoured and unarmoured targets had already indicated that the introduction of a weapon of at least 20 mm calibre was urgently needed.[96] A variant on the Spitfire design with four 20 mm Oerlikon cannon had been tendered to specification F37/35 but the order for prototypes had gone to the Westland Whirlwind in January 1939.[

• Crew: one pilot
• Length: 29 ft 11 in (9.12 m)
• Wingspan: 36 ft 10 in (11.23 m)

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, there were numerous problems which 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.[1] The first and most immediate problem was that the main Supermarine factory at Woolston was already working at full capacity fulfilling orders for Walrus and Stranraer flying boats. Although outside contractors were supposed to be involved in manufacturing many important Spitfire components, especially the wings, Vickers-Armstrong (the parent company) was reluctant to see the Spitfire being manufactured by outside concerns and was slow to release the necessary blueprints and subcomponents. As a result of the delays in getting the Spitfire into full production, the Air Ministry put forward a plan that production of the Spitfire 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 the problems could be overcome and further orders were placed for 200 Spitfires on 24 March 1938, the two orders covering the K, L and N prefix serial numbers.