Captain W.M. Jefferies was a 22nd century Starfleet officer, and one of the designers of the NX class starships. He was listed on the dedication plaque of the Enterprise NX-01 as a representative of the United Earth Space Probe Agency. (ENT: "Home"; Star Trek: Enterprise set artwork)

Jefferies served as an engineer on the NX Program, part of the effort to develop and refine the warp five engine. Lieutenant Charles Tucker III was among the officers assigned to Jefferies' team. (ENT: "First Flight")

While designing the NX-class, Captain Jefferies was a proponent of installing powerful weapons on the ship. Jonathan Archer, captain of Enterprise, once had an argument with Jefferies regarding this topic, asserting that he did not want to be captain of a warship attempting to make peaceful contact with other species. However, upon his return from the Delphic Expanse, during which Enterprise engaged in battle numerous times, Archer admitted that Jefferies had probably been correct. (ENT: "Home")