It travels from U.S. Route 34 (US 34) in Oswego north to US 12, near the Wisconsin state line, just south of Richmond.

It parallels IL 25, which travels along the eastern bank of the Fox River.

IL 31 between Aurora and Geneva was considered part of the Lincoln Highway transcontinental route.

[2] U.S. Route 430 (US 430) was commissioned from 1926 to 1935 in the U.S. state of Illinois, and traveled from Aurora to Crystal Lake.

[8] A bypass of McHenry continues to appear in documents planning for a Metra station at Prairie Grove, that include an interchange with the existing IL 31 alignment and the proposed west McHenry bypass alignment to the north of Gracy Road.