The origins of the class come from the Eastern Region declining an offer of allocations of the Peak class because they had reservations about the 1Co-Co1 wheel arrangement and they also preferred a locomotive of a greater horsepower. The Eastern Region had developed a good relationship with Brush during the production of the type 2 to 4 A1A-A1As (subsequently class 31) and this spurred the company to develop a new locomotive for them based around the Peak class equipment. It was the uprating of the Sulzer engine used in the Peaks to 2750 hp that allowed the project to gain momentum not only with Brush but also with other manufacturers competing for the large order of what was emerging as being about 500 locomotives. Brush emerged as the strongest contender and they offered BR a range of alternative designs, some, but not all, based on existing Peak class equipment. A Co-Co design with a new flat fronted load bearing body but using the existing Peak class equipment package was selected. In fact the first 20 used equipment originally intended for part of the final batch of the Peak class.

The agreement between BR and Brush included construction of the design using Brush electrical equipment under licence in BR works. In total 512 locomotives were built, 310 by Brush at Loughborough and 202 at BR's Crewe works.

photograph by Colin Duff

photograph by Colin Duff

photograph by Jonathan Hall

photograph by Jonathan Hall