In 1938, the southern end of US 31 was given a second designation when US 33 was extended into the state from Indiana to terminate in St. Joseph. Later that year, the US 31A in the Holland area was decommissioned. The next year, the US 31 concurrency was removed from US 2 in the Upper Peninsula and the former highway no longer crossed the Straits of Mackinac, terminating instead in Mackinaw City. By the early 1940, the Muskegon Bypass was given the US 31 designation, and the route downtown was redesignated US 31A.