The XB was one of several Pacific and Mikado locomotives designed in the Indian Railway Standard (IRS) programme for use in all India. Loosely based on the BESA principles, it had modern features and was built to serve growing traffic needs despite poor coal and mediocre rails. With an axle loading of 17 tons, it lay between the XA and XC.

99 were delivered to several railways, but soon several problems became evident. The most severe problems were many frame fractures and derailments. Additionally, the coupling rods often broke and the boiler had many problems in its design, so severe that the locomotives were not even able to haul a 350-ton-train at 60 mph (97 km/h) on the level.

With the help of the LMS in Britain, these problems were addressed by moving the trailing axle backwards, modifying the suspension and reducing the top speed. After this, they served fairly well and were in service into the eighties. None survived in India, but one is preserved in Pakistan at Lahore.