Second only to the Sopwith Camel in reputation as the RFC's outstanding fighter of World War I, the S.E.5 was designed under the direction of H.P. Folland. Of classic tractor biplane configuration, the S.E.5 was initiated to take advantage of the new Hispano-Suiza engine that began test-running in Spain in February 1915 and was in production in France a few weeks later. Two versions of the engine became available during 1916, the basic direct-drive 150hp unit and a geared version producing 200hp. Examples of both were included in the British orders placed in France and, subsequently, with Wolseley for licence-built examples (as the 150hp Python and 200hp Adder respectively). The S.E.5 was intended, from the outset, to be powered by the 200hp geared engine and to be armed with a 7.7mm Lewis machine gun firing through a hollow propeller shaft, but, in the event, early aircraft had to use the 150hp Hispano 8Aa, and had an armament of one Vickers gun in the front fuselage, offset to port, with interrupter gear, and a Lewis on a Foster mount above the centre section. Unarmed, the first of three prototypes of the S.E.5 flew on 22 November 1916. It was a compact single-bay biplane with equi-span wings featuring raked tips, a similarly-raked tailplane, triangular fin and almost rectangular rudder, with a small ventral fin and a V-strut undercarriage. A large windscreen was provided over the front of the cockpit. All major components were of conventional wood construction, with fabric covering. Of two further prototypes, one was similarly powered and first flew on 4 December 1916, whereas the other introduced the 200hp engine and became, effectively, the prototype for the S.E.5a. Production of the S.E.5 was ordered "off the drawing board" with a first batch of 24 built by the RAF at Farnborough, where the first was completed in March 1917. A second batch of 50 followed on, but at least 15 of these were to emerge as S.E.5a's, and some S.E.5s in service were also modified to have 200hp engines. In service with the RFC in France by early 1917, production S.E.5s were modified in various ways, particularly by removal of the windscreen. Other changes tried out on S.E.5s to improve the lateral control were consolidated in the S.E.5a.