| State: | MI |
| Type: | M 1919 |
| Route: | 31 |
| Formed: | [1] |
| Length Mi: | 166.910 |
| Map Custom: | yes |
| Map Notes: | M-31 highlighted in red on a modern map |
| Direction A: | South |
| Terminus A: | in Port Huron |
| Junction: | in Bad Axe |
| Direction B: | North |
| Terminus B: | in Saginaw |
| Counties: | St. Clair, Sanilac, Huron, Tuscola, Bay, Saginaw |
| Previous Type: | US-Bus |
| Previous Route: | 31 |
| Next Type: | M |
| Next Route: | 32 |
M-31 was a state trunkline highway in the Lower Peninsula in the US state of Michigan. It generally ran north from Port Huron along the Lake Huron shoreline through The Thumb region before turning inland. The highway crossed The Thumb and then ran along the Saginaw Bay shoreline before running inland again, terminating at Saginaw. It was one of the original state highways signposted in 1919, but it was renumbered as other highways in 1926, decommissioning the designation in the process. Parts of its route are now M-24, M-25, M-81 and M-142.
M-31 started at M-21 in Port Huron and ran northward along the Lake Huron shoreline. Along the way, it intersected the western terminus of M-46 before reaching Harbor Beach. In town, the original M-27 merged in from the north, and M-27/M-31 ran concurrently westward, turning inland. The two highways separated north of Ruth as M-27 turned southward. M-31 continued across The Thumb through Bad Axe, where it ran concurrently with M-19 in town. 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. The road passed through Sebewaing to Unionville before turning back inland. Running southward to Akron, the highway turned alternately westward and southward to Fairgrove. 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.[2]
When the state highway system was first signed in 1919,[3] M-31 was one of the original trunklines, originally running northward from Port Huron to Harbor Beach and then westward to Saginaw.[1] When the U.S. Highway System was approved on November 11, 1926,[4] M-31 was decommissioned in favor of alternate numbers. From Port Huron north to Harbor Beach, M-29 was extended as a replacement. The segment west to Bay Port was renumbered M-83 while from Bad Axe west it was also additionally part of M-29 to Unionville. The remainder was numbered M-84 from Unionville to Reese, and M-81 from Reese to Saginaw.[5]
