The highway carried on westward through Elkton and Pigeon to Bay Port. Once there, the trunkline turned southwesterly to follow along part of the Saginaw Bay.
M-31 next ran west along Bradleyville Road to a connection with M-81; the two highways ran concurrently south through Gilford before M-31 separated and turned back westward through Reese to Saginaw. The northern terminus in downtown Saginaw was at an intersection with what was then M-10.