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.
On 15 June 1936, locomotive No. 4009 was hauling an express passenger train which was in a rear-end collision at Welwyn Garden City, Hertfordshire due to a signalman's error. Fourteen people were killed and 29 were injured.
On 8 March 1937, locomotive No. 126 was hauling a passenger train that was derailed at Langrick, Lincolnshire due to the condition of the track.
On 25 August 1956, locomotive No. 61846 was hauling an empty stock train which ran away and crashed through the buffers at Filey Holiday Camp station, Yorkshire. The accident was due to the failure to connect the brake pipe between the train and locomotive.
The original ten locomotives were numbered 1000–1009 by the GNR, and became LNER 4000–4009. Those built for the LNER were numbered haphazardly, filling in gaps in the LNER's numbering scheme. In the LNER's 1946 renumbering programme, the K3s and K5 were renumbered 1800–1992, and they later became British Railways 61800–61992.