The fourth prototype was completed with the same fuselage structure as the second prototype and had provision for armament, with a Scarff ring mounting in the nose, a pair of post mountings in the mid position and a gun mounting in the rear fuselage. This was also the first O/100 to be fitted with Rolls-Royce Eagle engines. After completing acceptance trials, the second and third prototypes were retained at Manston to form a Handley Page training flight. The first prototype was rebuilt to production standard and the fourth prototype tested a new nacelle design, which was un-armoured, had an enlarged fuel tank and a shorter fairing obviating the need for the tail to fold. The new nacelle was used on all aircraft built after the initial batch of twelve. From "1461", an additional fuel tank was fitted in the fuselage above the bomb floor. A total of 46 O/100s were built before being superseded by the Type O/400.