 FM 400 was originally designated on June 25, 1945; its original routing was exclusively in Lubbock County, from Idalou northward to the Hale County line.On July 9 of that same year saw an extension northward, into Hale County and to a junction with FM 54 west of Petersburg.The road was lengthened, this time to the south, on December 16, 1948, from Idalou to a county road just north of the Slaton Airport (12.2 miles south of Idalou).