[6] The second SH 51 was designated on April 5, 1932, as a route from Dalhart northward to the Oklahoma state line.

[11] On February 4, 1941, the section of SH 51 from the Hockley–Terry county line to Brownfield was cancelled, creating a gap.

On May 14, 1941, the section from the Hockley–Terry county line to Brownfield was restored, partially closing the gap.

On September 22, 1942, to section of SH 51 from Levelland to the Hockley–Lamb county line was restored, closing the gap.

On October 24, 1956, the highway was rerouted southwest over FM 1214 from US 290 to US 90 and SH 227 from US 90 to the entrance of Big Bend National Park.