The Dash 8-40B, or B40-8, was introduced in 1988 after the Dash 8-39B. As its designation suggests, it has an output of 4,000 hp and two two-axle trucks. So it was 100 hp more powerful than its predecessor and also had revised trucks. Together with its six-axle sister, it was the first GE locomotive with microprocessor-equipped engine control units. It was also the basis for the P40DC “Genesis” passenger locomotive.

Of 151 B40-8 built, virtually all went to four customers: the St. Louis Southwestern, the ATSF, Conrail and the New York, Susquehanna & Western. The Dash 8-40BW or B40-8W was a variant with a wide cab. The ATSF was the only customer and ordered 83 for high-speed intermodal service. One prototype was rebuilt from a B39-8E, bringing the total number of narrow and wide cab variants to 235.