In World War II, Edward Thompson designed a new mixed-traffic 4-6-0 for the LNER that was to be called class B1. Due to wartime restrictions, its production costs had to be as low as possible thanks to the use of components from existing locomotives. So the boiler was derived from class B17 “Sandringham”, but had a larger firebox and higher pressure. Additional savings had been achieved by the use of two cylinders instead of the three of earlier LNER locomotives.

In the war, only small numbers were produced in the LNER's Darlington and Gorton works. After the war, more were ordered from commercial manufacturers. By 1952, North British had completed 290 and Vulcan Foundry 50 more, bringing the total number to 410. They were called “Bongos” after the name of No. 8306, but also “Antelopes” since many were called after types of antelopes and similar animals. They were used on most of the LNER network and also the later BR Eastern Region and were withdrawn between 1961 and 1967.