 DE 20 was designated to run from the Maryland border in Reliance east to US 113 in Millsboro by 1936, following its current alignment to Seaford before heading south along what was then US 13 (Front Street) to Blades, where it turned east onto High Street and picked up its current alignment to Millsboro. In 1939, plans were made to eliminate the grade crossing with the Pennsylvania Railroad (now Norfolk Southern's Delmarva Secondary) in Seaford by replacing it with a bridge over the tracks. Construction on this bridge began a year later. On September 2, 1941, the DE 20 bridge over the railroad tracks in Seaford opened to traffic.
