In 1915, Henry Folland of the Royal Aircraft Factory designed a new single-engined scout aircraft, the S.E.4a. While it had a similar designation to Folland's earlier Royal Aircraft Factory S.E.4 of 1914, which had been designed to be the fastest aircraft in the world, the S.E.4a was fundamentally a new aircraft, intended to investigate the relationship between stability and manoeuvrability, and for possible operational use.
The resulting design was a single-engined, single bay biplane. The fuselage structure was of mixed construction, with a steel tube forward section and a wooden box-girder rear section. The first prototype's fuselage was smoothly faired out to a circular section using formers and stringers, with the forward fuselage back to the cockpit covered in metal skinning and the rear fuselage fabric covered. The wood and fabric single-bay wings, unlike the S.E.4, had noticeable stagger between the upper and lower wings, but were fitted with similar, full span control surfaces which could be moved differentially as ailerons or together as camber changing flaps, to those used on the S.E.4.
The first prototype's engine, an 80 hp (60 kW) Gnome rotary engine, was mounted within a smooth cowling driving a two-bladed propeller fitted with a large, blunt spinner. This was found to lead to engine overheating and was replaced by a more conventional arrangement.
The remaining three prototypes had simpler structures, with flat-sided fuselages, and many of the drag reducing features of the first prototype omitted. They were powered by a range of engines of similar power to that used in the first prototype, including Clerget and Le Rhône rotaries.