The New York Times reports that “the first AR-15s were designed in the 1950s by Eugene M. Stoner, a Marine and inventor, who developed the weapon to military standards and for military service.”  

“It was an atypical rifle for its time, seemingly futuristic, and made partly with lightweight plastics and aluminum that traditionalists scorned. It fired a small-caliber, high-velocity bullet — the .223 — that was also considered revolutionary. The rifle was capable, via a selector lever, of semiautomatic or automatic fire.”  

“In the 1960s, under Defense Secretary Robert S. McNamara, the Pentagon bought vast quantities of the rifle, calling it the M-16, for American ground troops in Vietnam. The M-16’s firepower and reputation for lethality were necessary, in Mr. McNamara’s view, to counter the Kalashnikov assault rifles carried by the North Vietnamese Army and Vietcong.”  

The Washington Post adds: “Chronicled extensively in New York Times reporter C.J. Chivers’s book “The GUN,”  the AR-15, and eventually the M-16, was introduced as a replacement to the U.S. military’s M-14, a long high-caliber rifle based on an older World War II design. A small number of AR-15s were first bought by the Air Force in 1962 after a bit of salesmanship by Colt Firearms executives (Colt bought ArmaLite in 1959), that involved a pair of exploding watermelons and a general who disliked the M-14. With the Air Force’s initial purchase, the AR-15 entered the U.S. military’s arms procurement pipeline.”