The Class 40 was designed and built by English Electric. It has two extra pairs of carrying wheels on the outside of the six driving wheels on each bogie, giving it a 1-Co Co-1 wheel arrangement. It was part of the British Railways Modernisation Plan to built 2,000hp passenger express locomotives and the order was won by English Electric. They were ordered in several batches which finally totalled 200. Construction started in 1956 and the new locomotive was unveiled in the spring of 1958. Initially they were allocated to the Eastern, London Midland and Scottish Regions.

The first 134 locos built were equipped with disc codes and the next 22 locos had split code boxes, instead. From early in 1961, all new class members had a single codebox at each end, as the communicating doors in the cab fronts were no longer considered necessary. Some of the class received names of ocean going ships. The livery they carried until 1968 was BR Brunswick green but, after that, they were repainted in Rail blue as they passed through the works.

The class was successful and survived until 1985, after which they were considered to be obsolete. The first of the class was the last to go, in 1988. Seven Class 40s have survived in preservation.