May 13, 1864, 21-year-old Private William Christman of the 67th Pennsylvania Volunteer Infantry Regiment, who had died of peritonitis, became the first military man buried at Arlington.
More interesting details here:
file:///C:/Users/thomas%20j.%20turick/AppData/Local/Microsoft/Windows/INetCache/IE/QW28N84K/WilliamChristmanHistory.pdf