Tanks became a true terror for infantry in WWI, although an "antidote" for these steel monsters was quickly found. The first stage was introduction of new ammunition for existing weapons: armour piercing bullets, rifle grenades, bombs, mines. Special anti-tank rifles and cannons were built later, but grenades and bottles with incendiary fluid became some of the most common weapons used against tanks.
A Soviet tank destroyer squad at the start of the Great Patriotic War consisted of 2-3 men: 1-2 with Molotov cocktails and hand grenades, plus one with a submachine gun or machine gun. Vehicles could also be fought with a bundle of 3-5 fragmentation grenades tied together, although anti-tank grenades became the primary method for defeating enemy tanks.
"On October 2nd, 1942, 7 German tanks with submachine gunners attacked the front line of our defenses near the Red October factory in Stalingrad. Our forces were outnumbered. Red Armyman Panikako took grenades and bottles filled with KS fluid and headed for the lead tank. An enemy submachinegunner shattered one of his KS bottles and comrade Panikako burst into flame. Then, comrade Panikako jumped on the lead tank, burning it along with its crew. Comrade Panikako died the death of heroes."
"To improve the penetration of handheld anti-tank grenades by raising the amount and quality of explosive filler is economically infeasible and reduces the maximum range at which they can be thrown by hand. The penetration can be improved by taking advantage of the directed effect caused by the geometric shape of the charge, its position relative to the plane of the armour, shape and size of the fuse, and other reasons."
Thus the USSR began working on its first HEAT grenades. KB-30 tested its prospective designs against 60 mm thick armour plates. The Artillery Committee sent out the tender for directional anti-tank grenades on August 26th, 1942. The battle for the honour of designing the Red Army's first HEAT grenade broke out between KB-30 and NII-6.
KB-30 developed the RPG-42 ND "Comet" directional grenade. The 280-285 mm long grenade weighed 950 grams, contained 600 grams of explosive. Variants with a fuse in the handle as well as a screw-on fuse were tested. KB-30's grenades were tested from October 15th to October 17th, 1942. Trials showed that the RPG-42 grenade was simple to produce, but its size and weight didn't match the blueprints, the quality of the explosive filler was low, the safety was unreliable, and the penetration varied between 45 and 55 mm. Subsequent trials with improved grenades held on October 19th-21st showed that the penetration was still insufficient: 45 mm if thrown and 55-60 mm if detonated statically. Despite this, the NIPSVO considered the grenade as having passed trials and recommended it for military trials.