While the Tiger VIII engine used in the Whitley Mks II and III was more reliable than those used in early aircraft, the Whitley was re-engined with Rolls-Royce Merlin engines in 1938, giving rise to the "Whitley Mk IV". Three Whitley Mk I aircraft, "K7208", "K7209", and "K7211", were initially re-engined to serve as prototypes. The new engines are accredited with producing greatly improved performance. Other changes made included the replacement of the manually operated tail and retractable ventral turrets with a Nash & Thompson powered turret equipped with four .303 in (7.7 mm) Browning machine guns, the increasing of fuel tankage capacity, including two additional fuel tanks in the wing. A total of 40 Whitley Mk IV and Whitley Mk IVA, a sub-variant featuring more powerful models of the Merlin engine, were completed.