US 31 proper begins at a junction with US 90 and US 98 in Spanish Fort and exits the state into Tennessee running concurrently with Interstate 65 (I-65) near Ardmore.

Today, aside from portions through major towns and cities, US 31 is largely a rural two-lane highway.

About eleven miles later, the route leaves the Gulf Shores Parkway right-of-way and follows a town square around the Baldwin County Courthouse in Bay Minette.

Leaving Bay Minette, the route winds northeast, passing fairly close to the northwestern corner of Florida.

Upon crossing into Escambia County, the route winds east through Atmore before dipping south into Flomaton.

At this junction, the route dips down out of the viaduct onto surface streets, passing through the northern suburbs of Birmingham.

Winding through Fultondale, Gardendale, Morris, and Warrior, the route serves the communities of hilly northern Jefferson County.

A few miles later in Smoke Rise, the route leaves Interstate 65 at a trumpet interchange.

Following an arc, the route winds through a ridge cut, passes through Bangor, and crosses the Mulberry Fork of the Black Warrior River into Cullman County.

Immediately entering Garden City, the route slowly starts to arc back towards I-65.

Crossing the Tennessee River into Limestone County, the route has a Directional-T Interchange and loses U.S. 72 Alt, heading north out of the junction.