The portion of US 13 between Delmar and Dover was constructed as a state highway during the 1920s. Between Dover and Wilmington, the route was built as part of the cross-state DuPont Highway, which was completed in 1923 and improved transportation between northern and southern Delaware. North of Wilmington, what would become US 13 was originally built as the Philadelphia Pike in the 1820s and improved to a state highway by 1920. US 13 was designated through Delaware when the U.S. Highway System was created in 1926. The route was widened into a divided highway between Dover and Wilmington in the 1930s and between Delmar and Dover in the 1950s. US 13 was routed to bypass Dover in the 1950s. In 1970, US 13 was moved to its current alignment between Wilmington and Claymont on a bypass built in the 1930s. The portion of US 13 between Dover and Wilmington saw heavy traffic heading to the Delaware Beaches in the summer, which led to the construction of a freeway "Relief Route". This freeway was built as DE 1, which was completed in 2003. The construction of DE 1 necessitated the realignment of US 13 in two places.