Resembling a barrel with a pair of ironing boards for wings, the American-designed and -built Grumman Martlet I was the first aircraft in British service armed with medium-calibre guns. These aircraft were taken over from a French order, and at the Air Ministry’s behest the armament of six rifle-calibre guns was changed for four of medium calibre.

The 15 British-built Hawker Hurricanes that were in active service with the Belgian Air Force in the spring of 1940, one of which is seen here, were fitted with four 0-303in guns per wing. The Belgian-built aircraft were fitted with two 13-2mm FN guns per wing. Only one of the Belgian-built examples was delivered before Belgium fell, however, and was allocated to the test centre at Evere, so it is unlikely it saw action.

Arming an RCAF North American Mustang I in 1944. When the Mustang I was introduced into RAF service in the spring of 1942, it was fitted with a mixture of American “30-cal" (not British 0-303in) and “50-cal” Colt machine-guns; one 0-5in and two 0-3in guns in each wing and a pair of cowling-mounted 0-5ins which fired through the propeller arc, seen here.

A USAAF North American A-36 being re-armed in North Africa in 1943 with medium-calibre “50-cal” ammo; the British were then still using 0-303in machine-guns.

The wing gun bay of a Mustang I, showing the two smaller “30-cal” machine-guns and their feeds outboard (i.e. to the left) of the larger “50-cal”. The much larger size of the 0-5in round is apparent, as is the awkwardness of arranging the ammunition supply in the available space with contemporary solid (i.e. inflexible) feed-chutes.

In March 1943 the RAF evaluated the North American A-36, the dedicated ground-attack version of the Mustang, one example, EW998, being put into British markings for testing.The A-36 replaced the Mustang I’s mixed 0-3in/0-5in armament with six 0-5in Browning machine-guns; two in each wing and two in the cowling. The type was not adopted for RAF service, however.

In marked contrast to the cramped, untidy wing armament layout of the Mustang I, the three 0-5in machineguns mounted in each wing of the RAF’s Merlin-powered Mustang Mk IVs (which had no cowling-mounted guns), were fitted with flexible feed-chutes, allowing the guns to be mounted upright rather than at an awkward angle.

The Fairey Fulmar entered service with the Fleet Air Arm in June 1940, the first of the Service’s fighters to have eight (0-303in) guns. Late Fulmars, including the comparatively few Fulmar NF.2s, of which one is seen here, were fitted with an updated armament of four 0-50in guns, making them the only British fighter in FAA service to use the medium-calibre gun during the Second World War.