In 1912, a team at the Royal Aircraft Factory, led by Geoffrey de Havilland, started design of a single seat scout, or fast reconnaissance aircraft, the first aircraft in the world specifically designed for this role. The design was a small tractor biplane, and was named the B.S.1 (standing for Blériot Scout) after Louis Blériot, a pioneer of tractor configuration aircraft. It had a wooden monocoque circular section fuselage, and single-bay wings. Lateral control was by wing warping, while the aircraft was initially fitted with a small rudder without a fixed fin (a scaled down version of that fitted to the B.E.3), and a one-piece elevator. It was powered by a two-row, 14-cylinder Gnome rotary engine rated at 100 hp (75 kW).
The B.S.1 was first flown by Geoffrey de Havilland early in 1913, demonstrating excellent performance, with a maximum speed of 91.7 mph (147.6 km/h), a stalling speed of 51 mph (82 km/h) and a rate of climb of 900 ft/min (4.6 m/s), despite the engine only delivering about 82 hp (61 kW) instead of the promised 100 hp. De Havilland was not satisfied with the control afforded by the small rudder and designed a larger replacement, but on 27 March 1913, before the new rudder could be installed, he crashed the B.S.1, breaking his jaw and badly damaging the aircraft.
Following this accident, it was rebuilt, with an 80 hp single-row Gnome and new tail surfaces, with triangular fins above and below the fuselage, a larger rudder and conventional divided elevators. While the rebuilt aircraft was initially designated B.S.2, it was soon redesignated S.E.2 (for Scout Experimental). It was flown in this form by de Havilland in October 1913.
In April 1914, the S.E.2 was again rebuilt, this time under the supervision of Henry Folland, as de Havilland had left the Royal Aircraft Factory to become chief designer of Airco (the B.S.1/S.E.2 was the last design de Havilland produced for the Factory). The tail surfaces were again revised, with a larger fin and rudder, with new tailplane and elevators. The monocoque rear fuselage, which had been criticised as too expensive for mass production, was replaced by a conventional wood and fabric structure. Better streamlined struts were fitted, as well as streamline sectioned bracing wires (Raf-wires). It was first flown in this form on 3 October 1914. This modified version is often referred to as the "S.E.2a" - this designation was not used at the time, and was probably not official.