The early examples had a nose turret and rear turret, both being manually operated and mounting one Vickers 0.303 machine gun. On the Whitley Mk III this arrangement was substantially revised: a new retractable ventral 'dustbin' position was installed mounting twin .303 in (7.7 mm) Browning machine guns and the nose turret was also upgraded to a Nash & Thompson power-operated turret. On the Whitley Mk IV, the tail and ventral turrets were replaced with a Nash & Thompson power-operated turret mounting four Browning machine guns; upon the adoption of this turret arrangement, the Whitley became the most powerfully armed bomber in the world against attacks from the rear.