The machines were produced from 1963 onwards from the North British Type 2 (later Class 21 under TOPS) by replacing the original unreliable licence-built MAN engines of the Class 21s with Paxman Ventura V12 engines at Paxman's Colchester works.

The first unit to be re-engined was D6123, a further 19 machines were re-engined in 1965–1967 at Glasgow Works and mostly Inverurie Works, along with other modifications including the fitting of four-character headcode displays in the nose ends (D6123 retained its original front ends and so did not receive a headcode panel).

[2] Although these offered more power and much improved reliability over the original Class 21s, they did not survive much longer, due to their small class size and the use of a non-standard high-speed diesel engine.

The rebuilt locomotives were numbered:[3] In 1978 Hornby Railways launched its first version of the BR Class 29 in BR green and blue liveries in OO gauge.

[4] Dapol have more recently produced a Class 29 in multiple liveries.