Ostensibly designed under new Chief Mechanical Engineer (CME) William Stanier, they were in fact the last new design of the Midland Railway's school of engineering.

Although the last new Midland-style design, as subsequent Stanier engines incorporated much Great Western Railway practice, they were not the last MR-designed locomotives built with some 4Fs appearing as late as 1940.

The class was originally built with stovepipe chimneys, apparently due to an oversight by Stanier due to the design for future LMS locomotive chimneys not being finalised.

Two of the locomotives were fitted with vacuum control gear in 1934 for working the motor trains on the St Alban's branch, and allocated to Watford Junction shed.

[1] The remainder were fitted in the BR period and used at a number of different sheds including Warwick and Longsight.