The Class 31 was one of the Pilot Scheme locomotives ordered by British Railways to replace steam traction. They were originally built with Mirrlees JVS12T 1,250 bhp (930 kW) and 1,365 bhp (1,018 kW) engines and Brush electrical equipment, but the engines were not successful and in 1964 a programme of works commenced to re-engine the fleet with the 1,470 bhp (1,100 kW) English Electric 12SVT engines. The locomotives were originally known as Class 30 under TOPS, with re-engined examples joining Class 31. The class was originally intended for service on the Eastern Region, but gradually became common in both the Western and London Midland regions too.
Several sub-classes of Class 31 exist: