When the United States declared war on Germany on April 6, 1917, Mitchell was in Spain "en route" to France as an observer. He arrived in Paris on April 10, and set up an office for the Aviation Section from which he collaborated extensively with British and French air leaders such as General Hugh Trenchard, studying their strategies as well as their aircraft. On April 24, he made the first flight by an American officer over German lines, flying with a French pilot. Before long, Mitchell had gained enough experience to begin preparations for American air operations. Mitchell rapidly earned a reputation as a daring, flamboyant, and tireless leader. In May, he was promoted to lieutenant colonel. He was promoted to the temporary rank of colonel on October 10, 1917, to rank from August 5.