The LNER Thompson Class B1 is a class of steam locomotive designed by Edward Thompson for medium mixed traffic work. It was the LNER's equivalent to the highly successful GWR Hall Class and the LMS Stanier Black Five, two-cylinder mixed traffic 4-6-0s. However, it had the additional requirement of having to be cheap because, due to wartime and post-war economies, the LNER, never the richest railway company, had to make savings.

The prototype for the new B1 class 4-6-0 was built at Darlington and entered service on 12 December 1942. It was the first 2-cylinder main-line locomotive constructed for the LNER since the grouping, such had been Sir Nigel Gresley's faith in the 3-cylinder layout. With cost saving a wartime priority the LNER's draughtsmen went to great lengths to re-use existing patterns, jigs and tools to economise on materials and labour. Extensive use was made of welding instead of steel castings. The boiler was derived from the Diagram 100A type fitted to the LNER Class B17 Sandringham 4-6-0s but with a larger grate area and an increase in boiler pressure.

410 locomotives were built, but only two have been preserved, both of which were built by North British Locomotive Company. No. 61264 has the distinction of being the only LNER locomotive to be sent and later rescued from Barry Scrapyard.