In the First World War, the only suitable machine gun available for observers was a modified  infantry Lewis Gun. As the airstream was considered sufficient to cool the barrel, it's large cooling jacket was removed, and a magazine with nearly double the capacity was fitted, holding 97 instead of 47 rounds. In addition more efficient sights were devised, which allowed for deflection shooting. Although temperamental, it served well enough and remained in service until the mid 1930's.

However, the Air Ministry felt that a purpose designed gun would be of more use, so the Vickers Gas Operated gun was adopted. Also known as the 'K' gun, it was modified by Vickers Ltd from a French design, the Berthier. With internal parts redesigned and strengthened, and with a much improved rate of fire, it became standard issue for Air Gunners in the early years of the war. It used a 100 round drum magazine, and proved to be both reliable and rugged.

However, the introduction of powered turrets meant that a more compact design had to be considered, so the American .30 calibre M2 Browning was adopted, in slightly modified form as the .303 inch Mk II Aircraft Browning. With its reliable mechanism and high rate of fire, it was to become standard issue for all turret equipped aircraft. A solenoid was attached to permit the guns to be fired electrically. A bank of four guns was capable of delivering over 80 rounds per second - quite a formidable rate of firepower.

If the .303 Mk II had one major problem, it was lack of range. German fighters were usually equipped with cannon, which could easily outrange the Browning. Air Gunners usually had to wait until an enemy plane was within 400 yds before firing, by which time they would themselves have been under fire for several seconds. The 'Fifty Browning' was a development of the .30 infantry gun, but designed to fire the awesomely powerful .50 calibre round. It was the standard machine gun for all American aircraft, and by the end of the war was increasingly being fitted to British bombers. Its heavy weight (three times that of the .303 Browning) was offset by its range - effective to over 7000 yards, and the power of its bullet, easily capable of penetrating armour plate.