After the first 34 aircraft had been completed, the engines were replaced with the more reliable two-speed-supercharged Tiger VIIIs. "K7243", the 27th production Whitley, is believed to have served as a prototype following modifications. The resulting aircraft was designated as the Whitley Mk II. A total of 46 production aircraft were completed to the Whitley Mk II standard. One Whitley Mk II, "K7243", was used as a test bed for the 1,200 hp 21-cylinder radial Armstrong Siddeley Deerhound engine; on 6 January 1939, "K7243" made its first flight with the Deerhound. Another Whitley Mk I, "K7208", was modified to operate with a higher (33,500 lb) gross weight.