Introduced in 1924, the Type BX was a 2.5 tonner (2,540kg) fitting between the 2 ton (2,032kg) BT and 3 ton (3,048kg) X in carrying capacity. The BX was designed to provide a low-loading height for ease of load transfer.
Power from a 4,531cc AB/4 ioe straight 'four' was transmitted through a separate four-speed gearbox to the rear axle through a propeller shaft in the usual Thornycroft manner. Solid rubber tyres were fitted as standard, but pneumatics were available as extras. A departure from the BT chassis, which shared the BX's AB/4 engine, the BX's gearbox casing was an aluminium casting, and although slightly larger than the BT's gearbox it was considerably lighter. Having used the 2.5 ton (2,540kg) BX to displace the 3 ton (3,048kg) X, the BX was itself withdrawn in 1926 and replaced by 3 ton (3,048kg) KB, in response to customer feedback indicating renewed demand for a 3-tonner (3,048kg).