Since the creation of the first bullet-resistant vest in the late 1800s, numerous developments have been made in the field of soft body armor. Through World War I, World War II, and the Korean War, several vests were produced for the U.S. military. In 1969, American Body Armor was founded and began to produce a patented combination of quilted nylon and multiple steel plates to be marketed for the first time to the U.S. law enforcement community. By the mid-1970s, the DuPont Corporation introduced the Kevlar synthetic fiber, which was immediately incorporated into the evaluation program led by the National Institute of Law Enforcement and Criminal Justice (known today as simply the National Institute of Justice) to analyze the possibility of creating an everyday, concealable vest.
Since the original introduction of Kevlar, several new fibers and construction methods have been developed and proved effective. Armor is becoming lighter and more flexible, and continued improvements in technology are providing an increase in efficiency. Soft body armor functions on the principle of dispersing energy across an interlaced pattern of fibers. When a bullet strikes a vest, the horizontal and vertical fibers pull synchronously, absorbing the energy of the bullet and dispersing it over a wide area. This absorption of energy not only slows the bullet down to prevent penetration but also deforms the bullet, further limiting its ability to pierce the vest. In modern vests, it is not unusual for companies to incorporate two or three different types of fibers with different types of weave to maximize effectiveness.