What is now NY 347 was originally built by Suffolk County in the 1950s as two separate highways. Southwest of NY 25, it was known as the Smithtown Bypass and designated as CR 85. Northeast of NY 25, it was named the Nesconset–Port Jefferson Highway and designated as CR 80. Although the road acts as an eastward extension of the Northern State Parkway (with a section of Veterans Memorial Highway, now NY 454, connecting the two), Robert Moses planned an altogether different right-of-way for an extension of the Northern State. In fact, the Parkway was planned to have an interchange with the Smithtown Bypass west of CR 16 (Terry Road).
