After reviewing the existing privately commissioned Class 59, which was more powerful, highly reliable and with lower operating costs, EWS approached its builder Electro-Motive Diesel (EMD), then a division of General Motors. EMD offered their JT42CWR model, which had the same loading gauge-passing bodyshell as the Class 59. The engine and traction motors were different models to enable higher speeds, and the Class 66s incorporated General Motors' version of a 'self-steering bogie' ('radial truck', in American usage), designed to reduce track wear and increase adhesion on curves.

The initial classification was as Class 61, then they were subsequently given the Class 66 designation in the British classification system (TOPS). In 1998 Freightliner placed an order for locomotives. They were followed by GB Railfreight, and then Direct Rail Services. The last of more than 500 built over an 18-year period was No 66779, 'Evening Star', delivered to GB Railfreight in spring 2016.

Although sometimes unpopular with many rail enthusiasts, due to their ubiquity and having caused the displacement of several older types of (mostly) British built locomotives, their high reliability has helped rail freight to remain competitive. Rail enthusiasts labelled the type 'The Red Death' as they displaced many older types of locomotive while also acquiring the nicknames of 'Sheds' for the EWS (now DBS) locomotives (due to their upturned roof looking like a shed roof) with the Freightliner locomotives being called 'Freds' as a portmanteau of 'Freightliner and Shed'.

66432 has prominent history with its railtours, particularly the 'Cumbrian Crusader II' and 'The Cat & Dock', double heading each journey with 66843 and 68026 respectively.