Firearms made their entry at the end of the medieval era. When many knights and nobles purchased their new breast plates, they wanted
that the armor would protect them from early bullets. Armor makers would shoot the breast plates, and the resulting dent in the new armor was provided as evidence that it was
The oldest bullet-resistant fabric vests were made from silk. At the forefront, the Rev. Casimir Zeglen of Chicago, IL developed a bullet proof vest made of silk fabric at the turn of the last century. These expensive vests (often costing US $800 each in 1914. Equal to $15,000 in 2005) were capable of stopping relatively slow rounds from black powder handguns. On June 28, 1914, Franz Ferdinand, Archduke of Austria, heir to the Austro-Hungarian throne was wearing such a silk vest, but nonetheless died when shot in the neck above the vest with a .32 ACP bullet fired by Gavrilo Princip using a handgun, starting a chain of events that quickly escalated into World War I.
During World War I, the United States developed several types of body armor, including the chrome nickel steel Brewster Body Shield, which consisted of a breastplate and a headpiece and could withstand Lewis Gun bullets at 2,700 ft/s (820 m/s), but it was clumsy and heavy at 40 pounds (18 kg). Another type of body armor was designed in February 1918 by the Metropolitan Museum of Art. This breastplate was based on certain armor of the 15th century, weighed 27 pounds (12 kg), and was considered to be very noisy and restricting of the movements of the wearer. A scaled waistcoat of overlapping steel scales fixed to a leather lining was also designed; this armor weighed 11 pounds (5 kg), fit close to the body, and was considered comfortable.
During the late 1920s through the early 1930s, criminals in the United States began wearing less-expensive vests made from cotton padding and cloth. These early vests were capable of generally protecting against handgun bullets such as .22, .25, S&W .32 Long, S&W .32, .380 ACP, and .45 ACP traveling at slower speeds of up to approximately 1000 ft/s (300 m/s). This led to the development of the .357 Magnum cartridge for the use of law enforcement agents such as the FBI to overcome these vests.
In the early stages of World War II, some work was performed in the United States on designing body armor for infantrymen, but most models were too heavy, incompatible with existing equipment, and restricted the mobility of the wearer. For these reasons, development of infantry body armor was discontinued and attention was diverted to the development of "flak jackets" for aircraft crews. These flak jackets were made of nylon fabric and only capable of stopping flak and shrapnel, not the .38 Special or .357 Magnum bullets. The Japanese produced a few types of infantry body armor during World War II, but they did not see much use. Near the middle of 1944, development of infantry body armor in the United States restarted. Several vests were produced for the US military, including but not limited to the T34, the T39, the T62E1, and the M12.
There were several models of body armor in the Red Army, called SN-38, SN-39, SN-40, SN-40A, and SN-42. The number denotes the design year.
All were combat tested but only the SN-42 (in Russian language "Stalynoi Sagrudnik" SN=steel vest or CH-42) was put in production. It consisted of two pressed steel plates that protected the front torso and groin. The plates were 2mm and weighed 3.5kg. This armor was supplied to SHISBr(Assault Engineers) and Tankodesantnikam(Infantry that on rode tanks) of some Tank Brigades. Real combat experience showed that the MP-40 9mm bullet failed to penetrate at around 100-125m. It was very useful in dense, intense urban battles(Stalingrad) where the Germans used the MP-40 predominatly, but, because of its weight, was not practical for soldiers charging across an open field.