Body armor isn't anything new, I believe it was used as early as the 1920s - in a more modern form than the armor that knights used to wear into battle.

Richard Davis, who started Second Chance Body Armor (now out of business) paved the way for much of today's soft and heavy body armor. If memory serves me correctly, Davis started selling his soft Kevlar body armor in the early or mid-1970s. He made quite a name for himself, by actually demonstrating the effectiveness of body armor in live-fire demonstrations. He would fire a .44 Magnum handgun (with full power loads) into his own chest, while wearing his body armor. Needless to say, it was a very effective marketing tool.