This pretty class of engines were designed to replace the Wolverhampton '517' class, which by 1932 was looking well past their best. Charles Collett kept the design very much in the form of its predecessor but with modern refinements. The boiler was of the Group 7 SS type, still carrying a dome but with a pressure of 165 p.s.i., whilst the cab was fitted with large windows front and back. When introduced, the first class was numbered 4800 to 4874 with the later class numbered 5800 to 5819.