The Spitfire I weighed 5,280 lb. had a wing loading of 24 lb./s. ft. and a fuel capacity of 85 Imperial gallons. Its maximum speed was 362 mph its maximum diving speed was 450 mph its initial climb rate was 2,500 ft./min. and it took 9.4 minutes to climb to 20,000 feet. Its combat range was 395 miles and its roll rate was 140 deg./sec. Standard armament in what was subsequently to become known as the A wing was eight 0.303-in. Browning machine-guns with 300 rounds of ammunition. The speed of the Spitfire I was marginally higher than that of its principal opponent the Luftwaffes Messerschmidt 109f and it was infinitely more manaeuvrable than the German fighter although the Bf 109E could out climb and out dive the British fighter and its shell-firing cannon had a longer range than the Spitfire's machine-guns.
The 1,175 h.p. Merlin XII was adopted as the standard power plant in the Type 329 Spitfire II with a Rotol three-blade propeller and 73 lb. of amour protection but this variant was otherwise similar to the Spitfire I. Deliveries commenced in 1940 the Spitfire II having followed the Mark I on the production lines and becoming the first major production variant to be delivered from Castle Bromwich. In 1941 the Merlin 45 series of two-stage single-speed engines was adopted and the Type 349 Spitfire V so powered followed the Mark II into production and service.