Prior to 1963, part of the route, from the Bloomfield line to Route 75, was an unsigned but state-maintained road known as SR 905. Initially, the state had planned to turn over SR 905 to Windsor but when the final decisions for the 1962 Route Reclassification Act were announced, it was instead retained in the state highway system, with the continuation in Bloomfield also taken over by the state. At the beginning of 1963, SR 905 was renumbered as Route 305, officially becoming a signed route. Route 305 has had no significant changes since.