Some soldiers in the early North American colonies wore metal helmets and breastplates, but these proved cumbersome in the woodlands and were soon abandoned. Gunpowder cannon and small arms, and the increasing mobility of warfare, diminished the importance of personal armor, which finally disappeared by the end of the seventeenth century.
Steel helmets reappeared in the twentieth century largely to protect against shrapnel and fragments from artillery shells. In World War I, the Americans adopted the shallow British “tin hat,” but the U.S. Army developed its own deeper helmet for World War II. A new configuration, including more neck protection, was adopted in the 1980s.
The French and Germans experimented with metal cuirasses for machine gunners in World War I; the Americans did not adopt chest armor until World War II, when some bomber crews were provided with “flak jackets.”
Beginning in the Vietnam War, American combat infantrymen wore protective vests made of new composite materials, such as kevlar, covered by fabric. The vests, which provided relative flexibility and low heat retention, were designed to protect against blast fragments and antipersonnel, small‐arms “ball” ammunition. Crews of ground vehicles, helicopters, and other aircraft today wear heavier vests made of metal (often titanium), or ceramic tiles contained in the pockets; these prove bulkier but more efficient against small‐caliber armor‐piercing ammunition.
John Whiteclay Chambers II
"Body Armor." The Oxford Companion to American Military History. . Encyclopedia.com. (July 17, 2018). http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/body-armor
"Body Armor." The Oxford Companion to American Military History. . Retrieved July 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/body-armor