It wasn't. From Wikipedia
M-112 was the state highway designation given to segments of what is now Interstate 94 in the Metro Detroit area.
The 8.7 mile long "Willow Run Expressway" was completed in September 1942 to help workers in the Detroit area get to Willow Run on the eastern edge of Washtenaw County where the B-24 Liberator airplanes were made during the war. Built right next to the existing Chase Rd., it was hurriedly constructed after a change in construction priorities resulting from the bombing of Pearl Harbor in December 1941. It was built as a four-lane divided highway with some cross-road intersections, from Huron River Drive in western Romulus to US 112 on the west side of Willow Run via the Willow Run Bypass. Chase Rd. acted as the new service drive for the expressway. Prior to the United States entry into WWII, plans had been in the works to build a "Crosstown" highway through downtown Detroit into Macomb County which was built in the 1950s as the Edsel Ford Expressway.
From 1943 through 1945, the "Detroit Industrial Expressway" was built as a completely limited-access freeway from the eastern end of the Willow Run Expressway in Romulus first to Southfield Highway in Allen Park and then to US 112/Michigan Avenue (now US 12) near the boundary between Detroit and Dearborn.
Both the Willow Run and Detroit Industrial Expressways were designated as M-112, because it connected with US 112 on its eastern and western ends. All of M-112 was redesignated as US 12 in 1956, and the designation has not been used since. Beginning in 1958, the route was assimilated into the Interstate System as I-94. In the mid 1960s, the Willow Run Expressway was reconstructed to full Interstate standards, complete with full interchanges at Belleville and Haggerty Rds.
Some tri-level grade separation bridges were built as part of the Willow Run and Detroit Industrial Expressways that are recognized as eligible for the National Register of Historic Places as significant components of the expressway system and as creative engineering solutions to the massive volume of traffic anticipated when shifts changed at the bomber plant. They can be found along US 12 as it passes by the Willow Run plant, now called Willow Run Transmission.