There were always historical attempts to make bulletproof armour ever since the very first gunpowder weapons. In fact, the very term "bulletproof" comes from the practice of early modern armourers firing a bullet into their armor as proof that it could withstand bullets. The reason why the use of armor died out in Europe wasn't because armourers weren't capable of making armour capable of resisting bullets, so much as that equipping soldiers with armour grew prohibitively expensive as army sizes increased.