Frazer Nash was a British automobile manufacturer and engineering company founded in 1922 that had produced around 400 cars by the mid 1930’s. A separate company Nash & Thompson was established in 1929 by business partners Archibald Frazer Nash and Henry Ronald Godfrey to develop aircraft turret designs that Frazer Nash had originated. The designs were consequently numbered in a series prefixed with "F.N".
Nash & Thomson built a wide range of turrets for aircraft. All used hydraulic power supplied from the aircraft’s hydraulic system and carried 0.303 inch (7.7 mm) Vickers K or Browning machine guns. Later in the war heavier calibre 0.5 inch machine guns were introduced on some models. The same model of turret might be fitted to several different aircraft types, the F.N.5 for example being fitted to Lancasters, Wellingtons and Stirlings.
The F.N.4A is a four gun tail turret that was fitted to the Short Stirling and Armstrong Whitworth Whitley, with a different cupola design being fitted to each aircraft type, this example being Whitley. The Whitleys defensive armament consisted of two turrets, an F.N.16 nose turret, equipped with a single Vickers 'K' machine gun, using a 97 round drum-type magazine, and the F.N.4A tail turret, fitted with four 7.7 mm (0.303 in) Browning machine guns, using ammunition belts that supplied 1,000 rounds to each gun. The ammunition boxes in the F.N.4 A were fitted under the guns, which limited the supply and could affect the trim of the aircraft.