Were bulletproof vests used during World War II?

Answer

They were indeed made and used, by almost all belligerents, but only to a limited extent and for very specific troops. The reasons were the many technical limitations of the vests/armours: they were usually very heavy, limited freedom of movement, offered little protection against high-velocity projectiles (so against shrapnel, less against bullets), were expensive to produce.

The British and Canadians developed a vest made of layers of steel plate, which was mainly used by men in naval artillery, anti-aircraft defense and medical aid stations. Wilkinson Sword, known for razor blades, made a ‘flak jacket’, oddly enough, out of layers of nylon at the request of a military surgeon, Colonel Malcom C. Grow. Grow found that many aircrew died from injuries in the fuselage caused by shrapnel from the German FliegerAbwehrKanone (FlAK) and that a protective sleeveless, thick and tough vest could prevent a lot of suffering. In narrower British bombers the vest was inconvenient, but in the US Air Force Wilkinson’s flak jackets became standard equipment from 1943. The American infantry was simultaneously equipped with similar vests incorporating compressed and preformed fiberglass plates, the Doron plates. Neither the flak jackets nor the Doron jackets were really bulletproof, but they did offer protection against flying projectiles. And they were light and cheap. In photographs, they were mainly worn by aircrew and aircraft carrier crews, and resemble contemporary life jackets. The Soviets developed a steel vest, the SN-42 (Stalnoi Nagrudnik, Steel Breastplate), which was so heavy and clumsy that it was only used by deminers and infantry troops riding on tanks.