In March 1912, after assignments in the Philippines that saw him tour battlefields of the Russo-Japanese War and conclude that war with Japan was inevitable one day, Mitchell was one of 21 officers selected to serve on the General Staff—at the time, its youngest member at age 32. He appeared in August 1913 at legislative hearings considering a bill to make Army aviation a branch separate from the Signal Corps and testified against the bill. As the only Signal Corps officer on the General Staff, he was chosen as temporary head of the Aviation Section, U.S. Signal Corps, a predecessor of the present day United States Air Force, in May 1916, when its head was reprimanded and relieved of duty for malfeasance in the section. Mitchell administered the section until the new head, Lieutenant Colonel George O. Squier, arrived from attaché duties in London, England, where World War I was in progress, then became his permanent assistant. In June, he took private flying lessons at the Curtiss Flying School because he was proscribed by law from aviator training by age and rank, at an expense to himself of $1,470 (approximately $33,000 in 2015). In July 1916, he was promoted to major and appointed Chief of the Air Service of the First Army.