The 'Ace' was born in the air war of World War 1 - led by none other than German high scorer Manfred von Richthofen (The Red Baron).
With the arrival of the armed, single-seat fighter in World War 1 arrived the term 'Ace' used to describe a military aviator who had acrued multiple air kills - typically five or more. The term originated from French newspapers of the period when aviator Adolphe Pegoud was credited with downing five German aircraft. Various measures were had by the various countries involved in the war to ensure that an air kill was indeed legitimate. In time, even individual bomber crewman - who could prove adept at taking down enemy fighters as well - were also given the title of Ace.