W.M. Jefferies was a Human Starfleet officer in the 22nd century. He was one of the designers of the NX-class starships. (ENT: "Home") In the 2140s, as a captain, Jefferies was an engineer on the NX Program. This program was part of an effort to develop and refine the warp five engine. Lieutenant Charles Tucker III was among the officers assigned to Jefferies' team. (ENT: "First Flight")