Colt's Browning machine gun was chosen for the Spitfire in 1934 after competition with four other gun manufacturers, and it was to be made under license in the UK.
From "Spitfire The History", by Morgan and Shacklady:
"By strange oversight one of the largest weapons manufacturers, Birmingham Small Arms, was not sent a copy of the specification and when the error was discovered they were provided with a set of blueprints. In June 1935 they were asked to quote for the production of 1050 guns at the rate of 50 per week. James Leek, of BSA, at a meeting in the Air Ministry some time after the quote was received, expressed an opinion that a rate of 2000, not 50, guns per week would be necessary if the United Kingdom was ever to be called upon to meet the threat of Germany's military preparations.
. . . His advice was ignored, but he was later to be completely vindicated when his prediction that 2000 guns per week were necessary when that total was required, and ordered [from BSA?], in June 1940."
