A lot deoends on what you mean, Different types of body armor were developed by different people at different times. The first military body armor used in the 20th century was developed by the Germans, the British, and the French during World War One. THe German armo was the best and was widely used by the German SturmTruooen in 1918.
The first modern body armor was developed by the US Army and Marine Corps toward the end of World War Two. It was non-mettalic using ballistic Nylon. It was issued to US troops in Korea in 1953. It was not intended to be bullet proof but was designed to stop shell fragments, mortar fragments, and fragmnts from hand grenades.
Richard Davis pioneered modern soft body armor which he marketed as "Second Chance" armor. In the early days he used to convince sceptical police officers by shppting himself in the chest with a .38 Special to prove that it would stop the bullet. These demonstartions made a lot of true believers among Police officers who saw them!
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[This message has been edited by Hard Ball (edited March 25, 2000).]