On June 21, 1917, the department's Office of State Highway Engineer issued a proposal for the state highway system. The proposal included SH 20 originally planned as the Austin-Houston Highway between Austin and Houston by way of Bastrop, Giddings, Brenham, and Hempstead. On July 17, 1917, the planned route was extended from Austin to Brady by way of Burnet, Llano, and Mason. On December 17, 1918, SH 20 was rerouted away from Bastrop, with the section from Austin to Bastrop transferred to SH 3A. The section of SH 20 from Hempstead to Houston was cancelled as it was already part of the Gulf Division Branch of SH 2.