SSH 21A was added to the state highway system in 1937. This route began at Jct PSH 21 (now SR 3) at Poulsbo and headed in an easterly direction to Suquamish, then via a ferry to the north end of Bainbridge Island, then southerly to Port Blakely. The route description was changed in state law in 1949. Suquamish was dropped from the route although the present bridge replacing the ferry was not completed until the early 1950s. Winslow became the southern terminus of SSH 15A.
When the current state highway numbering system was first posted in January 1964, SSH 21A became SR 305.
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