The first production standard Halifax, the Mk.I, had a 22 ft (6.7 m) long bomb bay as well as six bomb cells in the wings, providing a bomb capacity of 13,000 lb (5,900 kg). Defensive armament consisted of two .303 in (7.7 mm) Browning machine guns in a Boulton Paul Type C nose turret, with an additional four in a Boulton Paul Type E tail turret. Some aircraft included two additional .303 in (7.7 mm) Vickers K machine guns in beam (side, or "waist") positions. Subtle modifications distinguished the Mk I aircraft. Aircraft of the first batch of fifty Mk I Halifaxes were designated Mk I Series I.