Under the British Railways Modernisation Plan, a batch of ten 1,000 hp (746 kW) diesel-electric locomotives were ordered from the North British Locomotive Co. for evaluation under BR's dieselisation pilot scheme. At the same time, six externally similar locomotives employing hydraulic transmission were ordered for comparison, these becoming Class 22. Repeat orders resulted in a total of 58 of the diesel-electric locomotives being built (numbered D6100–6157). They were delivered between December 1958 and November 1960.
The first 38 locomotives entered service in 1958-59 from the Eastern Region depots at Stratford, Hornsey and Ipswich engine shed on commuter services into London, where they were evaluated against rival designs from English Electric, British Railways, Birmingham RC&W and Brush. The type proved chronically unreliable in Eastern Region service - by March 1960 the Hornsey allocation had moved to New England Yard, Peterborough for storage. This came to the attention of the newspapers and the Daily Telegraph reported that brand new diesel locomotives were being hidden and dumped, which caused questions to be asked in Parliament. The D6100s moved north to the Scottish Region in mid-April 1960, ostensibly to be nearer to the NBL works for repairs but allegedly to move them away from the eyes of the national press.
The final 20 locomotives had uprated 1,100 hp (820 kW) engines and were delivered to Kittybrewster depot on the Scottish Region. They were joined on the Scottish Region by the first 38 locos, which were allocated to Glasgow Eastfield depot, close to the North British factory at Springburn where they had been built. They were used widely across the Scottish Region on a range of work, freight, local passenger and express passenger, the latter sometimes in pairs.
They proved to be unreliable in service, and during 1960 the Eastern Region fleet was transferred to Eastfield depot on the Scottish Region for convenience of return to their manufacturer when warranty work was required. However, the North British Locomotive Works closed in 1962, by which time the type's principal shortcomings had become plain. In common with many NBL designs of diesel locomotive for British Rail, the D6100s suffered from a poorly thought-out and badly executed design. In particular there were problems with the coupling between the power unit and the generator. The engines themselves were a MAN design, but which were built under licence by NBL and of inferior quality to the German originals. Engine cooling systems proved to be inadequate, diesel engines leaked and were not constructed to the appropriate tolerances, cylinder heads fractured and lubricating oil escaped into the battery compartments located below the power unit. The positioning of minor components within the locomotive bodyshell meant that small faults could only be rectified on depot or by return to a railway workshop, which resulted in poor daily availability for traffic figures for the type. Engine room fires were common and wrote off several locomotives.
In an attempt to improve reliability 20 locomotives, (D6100–03, D6106-D6108, D6112–D6114, D6116, D6119, D6121, D6123, D6124, D6129, D6130, D6133, D6134 and D6137), were re-engined between 1963 and 1965 with 1,350 hp (1,010 kW) Paxman Ventura engines to form Class 29.
The remaining 38 locomotives retained their original NBL/MAN engines until they were withdrawn from service between December 1967 and August 1968 and sold for scrap. Most were cut up by Scottish scrap dealers McWilliams of Shettleston or Barnes and Bell of Coatbridge, but locomotive D6122 was sold to Woodham Brothersscrapyard in Barry, Vale of Glamorgan, South Wales, where it languished until 1980 before being broken up. None have survived.