The final aircraft built at Farnborough against contracts for 74 S.E.5s placed in 1917 was used early the following year for an experimental programme aimed at improving the performance and fighting ability of the type. Fitted with a 200hp Hispano-Suiza 8B engine and armed with the Lewis and Vickers gun combination of the standard S.E.5a, it had new single-bay wings of unequal span and chord. In addition, a retractable radiator was provided in the forward fuselage, and a large, shallow, propeller spinner was fitted, to combine with a re-profiled cowling and give better streamlining of the fuselage. The head-fairing behind the cockpit was also improved. Tests in 1918 revealed little performance gain or handling benefit, with the extra drag of the big upper wing offsetting gains from the more streamlined fuselage. Standard S.E.5a wings were fitted to the S.E.5b in 1919, as well as a modified horizontal tail, for comparative testing at Martlesham Heath, and this aircraft made an appearance at the RAF Pageant at Hendon in 1920.