The most significant difference between the two types was the use of Eagle VIII engines. Unlike the earlier engines, this engine was not built in handed versions, because production of engines of both types for engine type approval had been difficult. Wind tunnel tests at the NPL established that the counter-rotating propellers causing directional instability with the O/100. Only one version was necessary, simplifying production and maintenance and the p-factor was overcome by slightly offsetting the fin. The O/400 had a strengthened fuselage, an increased bomb load, the nacelle tanks were removed and the fuel was carried in two fuselage tanks, supplying a pair of gravity tanks. The new nacelles were smaller and had simplified supporting struts; the reduction of drag improved maximum speed and ceiling. The revised nacelle was tested in "3188", which in 1917 was flown at Martlesham Heath with a variety of engine installations. An initial order for 100 of the revised design, with Sunbeam Maori or Eagle engines, was placed on 14 August but cancelled shortly afterwards. Twelve sets of Cricklewood-built components were transferred to the Royal Aircraft Factory, where they were assembled into the first production O/400s. More than 400 were supplied before the Armistice at a price of £6,000 each. Another 107 were licence-built in the US by the Standard Aircraft Corporation (out of 1,500 ordered by the air corps). Forty-six out of an order for fifty were built by Clayton & Shuttleworth in Lincoln.