The engines were initially designated as B class, being a development of the standard B class locomotives designed by Reid some eight years previously, and represented the culmination of a long evolution on the NBR of powerful 0-6-0 freight engines.

In 1914, Reid ordered the first batch of 5 engines of his improved B class design.

From 1914 to 1919, these engines were simply considered to be part of the B class, by which time around 70 had been produced.

Several of the earlier locomotives were upgraded to the new 175 psi boiler pressure, and redesignated as S class.

On some parts of the network (notably the West Highland Line), they were used for both freight and passenger duties.