The B class of the Swedish State Railways (SJ) was a type of steam locomotive for mixed traffic, introduced in 1909.

Most B class locomotives were eventually rebuilt with fully enclosed cabs, and some were equipped for oil-firing after the Second World War.

[2]: 106  Some other kinds of locomotives were later rebuilt with this type of boiler, creating the A2,[1]: 37  Ga2,[1]: 80  Ga4[1]: 82  and Mc[1]: 85  classes.

[1]: 20  This changed with rising international tensions: the B class locomotives were deemed to be "valuable in wartime conditions", and as such included in a 1938 refurbishment program.

[1]: 20 At least four locomotives were leased to the Norwegian State Railways during the German occupation of Norway.

[1]: 45  The shorter six-wheeled tenders made it possible to turn the locomotives on smaller turntables.

[2]: 76  The need for steam locomotives declined again in the post-war era, but a large number of surplus locomotives were preserved in the strategic reserve, kept in usable condition to replace diesel-powered vehicles if the import of oil was interrupted or electric vehicles if the power supply was destroyed.

[8]: 153  Three locomotives were soon transferred from BJ to close cooperator Södra Dalarnes Järnväg (SDJ),[7]: 32  but it is unclear if they were ever formally registered with that company.

[2]: 76 The B3 class differed from other locomotives of BJ, as the driver's position never was moved from the left to the right side.

[4] SWB did make some modifications on their locomotives: the sandboxes were moved,[8]: 156  and the blastpipes were modified, increasing the production of steam.

[8]: 155  Some minor improvements were made, including fully enclosed cabs and the use of roller bearings.

[8]: 157 The long use of the B class in the strategic reserve meant that a large number of the locomotives survived into the 1990s, and nearly half of them have been preserved.

[2]: 18  A few were sold nevertheless: B 1220 and 1289 are at the Zuid-Limburgse Stoomtrein Maatschappij (ZLSM) in the Netherlands,[6] and SWB A2 101 at Nene Valley Railway in the United Kingdom.