At the beginning of World War II, aircraft design had far outstripped the design of aircraft weapons. During the Battle of Britain, the famous Spitfire and Hurricane were equipped with eight .303-caliber Browning machine guns. The same .303 calibre gun that was used in the First World War. The British soon discovered that the .303 calibre machine gun, while reliable and accurate, lacked the necessary punch. After the Battle of Britain, the Spitfire's armament was increased to include two 20mm cannon and four .303 calibre machine guns. German and Japanese fighter armament also increased. As the bomber offensive against the Axis grew, Allied bomber crews were confronted with enemy fighters armed with 20mm and 30mm cannon. To defend themselves, American gunners relied on the hard hitting .50 calibre machine gun, while the British soldiered on with the reliable but light.303 calibre machine gun. The reliable .303 calibre (7.7 mm) Browning Machine Gun. This weapon was standard armament on all British heavy bombers. The famous .50 calibre (12.7 mm) Browning Machine Gun. This weapon was standard equipment in all American fighters and bombers.