The Anson Mills Building is a historic building located at 303 North Oregon Street in El Paso, Texas. The building stands on the original site of the 1832 Ponce de León ranch. Anson Mills hired Henry C. Trost of the Trost and Trost architectural firm to design and construct the building. Trost was the area's foremost pioneer in the use of reinforced concrete. Built in 1910–1911, the building was only the second concrete-frame skyscraper in the United States, and one of the largest all-concrete buildings. At 145 feet, the 12-story Mills Building was the tallest building in El Paso when completed. The architectural firm of Trost and Trost moved its offices to the building upon completion, where they remained until 1920. The Mills family sold the building in 1965. The building stands on a corner site opposite San Jacinto Plaza, with a gracefully curved street facade that wraps around the south and east sides. Like many of Trost's designs, the Anson Mills Building's overall form and strong verticality, as well as details of the ornamentation and cornice, are reminiscent of the Chicago School work of Louis Sullivan.