The first production Spitfire, the mark I was powered by a Merlin III giving 1,030 hp and a maximum speed of 367 mph, although in squadron service, with armour plate for pilot protection added, the top speed was more likely to be 345 mph. The first batch had a straight topped cockpit canopy, later changed for the bulged type. This is the mark that mostly fought in the Battle of Britain, (although some mark II machines were in service by September 1940). Originally fitted only with wooden two blade fixed pitch propellers, they were later equipped with de Havilland three bladed variable pitch airscrews. Armament was either eight Browning .303 calibre machine guns (Mk IA) or two 20 mm cannons and four machine guns (Mk 1B). The cannon version was not produced in great quantity, the early cannon installations were not very satisfactory and prone to the mechanism jamming. The Spitfire I entered service in 1938 with 19 and 66 Squadrons. In all some 1,566 Mk I Spitfires were produced.