The type was a more powerful development of the earlier H3 (LNER K2) class and was notable at the time, as the 6-foot-diameter (1.8 m) boilers were the largest fitted to any British locomotive to that date.

After formation of the London and North Eastern Railway, the type became known as class K3 and was adopted as an LNER standard design.

[1] The first ten locomotives were built at the GNR's Doncaster Works in 1920, to the design of Nigel Gresley.

They were excellent mixed-traffic locomotives, although their large size restricted their route availability.

The railway writer O. S. Nock described a journey on the footplate of a Mogul hauling a heavy express goods train in 1942.