In 1926, the United States Numbered Highway system was introduced, and the route across the Texas Panhandle was given the number of 66. It was designated along already created roads in the Texas highway network. Large portions of the highway remained earthen roads until the entire route was completely paved in 1938. A few realignments were made to the original route. A new routing on the east side of Amarillo was completed in 1959 to make way for the expansion of the Amarillo Air Force Base, while the rest of the route through Amarillo was rebuilt on the north side of downtown.
