An earlier B class of Double Fairlies had entered service in 1874, but as they had departed from the ownership of the New Zealand Railways (NZR) by the end of 1896, the B classification was free to be re-used.

The first was built in NZR's own Addington Workshops in Christchurch and entered service on 4 May 1899, and an order was placed with Sharp, Stewart and Company of Glasgow, Scotland to supply four more.

The locomotives were advanced for their time, featuring a new piston valve design and a modified form of Walschaerts valve gear, and they were designed to haul 600-long-ton (610 t; 670-short-ton) freight trains on flat lines and 220 long tons (224 t; 246 short tons) on the hilly section of the Main South Line between Oamaru and Dunedin.

Only a couple of years after their arrival in New Zealand, three of the four Sharp, Stewart models entered NZR's Addington and Hillside (Dunedin) Workshops to be rebuilt, emerging as the WE class 4-6-4T tank locomotives.

The frames flexibility was improved by removing the continuous running plates and replacing them with boiler mounted boards.

In their early years, the B class hauled freight trains between Christchurch and Dunedin, with their pulling power a considerable asset.

About the same time, these modifications were being carried out, NZR began extending the lives of specialised goods locomotives by adapting them for heavy shunting work.

By the 1950s, with most working on the West Coast, they were still averaging 20,000 miles,[2][page needed] making the class one of the most consistent performers for NZR.

All members of the B class, including the three converted into WE tanks, survived until the last decade of steam in New Zealand in the 1960s, with the country's last regular steam-hauled service running on 26 October 1971.