The PIAT was 39 inches (0.99 m) long and weighed 32 pounds (15 kg), with an effective direct fire range of approximately 115 yards (110 m) and a maximum indirect fire range of 350 yards (320 m). It could be carried and operated by one man, but was usually assigned to a two-man team, the second man acting as an ammunition carrier and loader. The PIAT launcher was a tube constructed out of thin sheets of steel, and contained the trigger mechanism and firing spring. At the front of the launcher was a small trough in which the bomb was placed, and the spigot ran down the middle of the launcher and into the trough. Padding for the user's shoulder was fitted to the other end of the launcher, and rudimentary aperture sights were fitted on top for aiming; the bombs launched by the PIAT possessed hollow tubular tails, into which a small propellant cartridge was inserted, and hollow-charge warheads.
At the beginning of the Second World War, the British Army possessed two primary anti-tank weapons for its infantry: the Boys anti-tank rifle and the No. 68 AT Rifle Grenade. However, neither of these was particularly effective as an anti-tank weapon. The No. 68 anti-tank grenade was designed to be fired from a discharger fitted onto the muzzle of an infantryman's rifle, but this meant that the grenade was too light to deal significant damage, resulting in it rarely being used in action. The Boys was also inadequate in the anti-tank role. It was heavy, which meant that it was difficult for infantry to handle effectively, and was outdated; by 1940 it was effective only at short ranges, and then only against armoured cars and light tanks. In November 1941 during Operation Crusader, part of the North African Campaign, staff officers of the British Eighth Army were unable to find even a single instance of the Boys knocking out a German tank.
Due to these limitations, a new infantry anti-tank weapon was required, and this ultimately came in the form of the Projector, Infantry, Anti-Tank, commonly abbreviated to PIAT. The origins of the PIAT can be traced back as far as 1888, when an American engineer by the name of Charles Edward Munroe was experimenting with guncotton; he discovered that the explosive would yield a great deal more damage if there were a recess in it facing the target. This phenomenon is known as the 'Munroe effect'. The German scientist Egon Neumann, found that lining the recess with metal enhanced the damage dealt even more. By the 1930s Henry Mohaupt, a Swiss engineer, had developed this technology even further and created hollow charge ammunition. This consisted of a recessed metal cone placed into an explosive warhead; when the warhead hit its target, the explosive detonated and turned the cone into an extremely high-speed spike. The speed of the spike, and the immense pressure it caused on impact, allowed it to create a small hole in armour plating and send a large pressure wave and large amounts of fragments into the interior of the target. It was this technology that was utilized in the No. 68 anti-tank grenade.
Although the technology existed, it remained for British designers to develop a system that could deliver hollow-charge ammunition in a larger size and with a greater range than that possessed by the No. 68. At the same time that Mohaupt was developing hollow-charge ammunition, Lieutenant Colonel Stewart Blacker of the Royal Artillery was investigating the possibility of developing a lightweight platoon mortar. However, rather than using the conventional system of firing the mortar shell from a barrel fixed to a baseplate, Blacker wanted to use the spigot mortar system. Instead of a barrel, there was a steel rod known as a 'spigot' fixed to a baseplate, and the bomb itself had a propellant charge inside its tail. When the mortar was to be fired, the bomb was pushed down onto the spigot, which exploded the propellant charge and blew the bomb into the air. By effectively putting the barrel on the inside of the weapon, the barrel diameter was no longer a limitation on the warhead size. Blacker eventually designed a lightweight mortar that he named the 'Arbalest' and submitted it to the War Office, but it was turned down in favour of a Spanish design. Undeterred, however, Blacker continued with his experiments and decided to try to invent a hand-held anti-tank weapon based on the spigot design, but found that the spigot could not generate sufficient velocity needed to penetrate armour. But he did not abandon the design, and eventually come up with the Blacker Bombard, a swivelling spigot-style system that could launch a 20-pound (9 kg) bomb approximately 100 yards (90 m); although the bombs it fired could not actually penetrate armour, they could still severely damage tanks, and in 1940 a large number of Blacker Bombards were issued to the Home Guard as anti-tank weapons.
When Blacker became aware of the existence of hollow-charge ammunition, he realized that it was exactly the kind of ammunition he was looking for to develop a hand-held anti-tank weapon, as it depended upon the energy contained within itself, and not the sheer velocity at which it was fired. Blacker then developed a hollow-charge bomb with a propellant charge in its tail, which fitted into a shoulder-fired launcher that consisted of a metal casing containing a large spring and a spigot; the bomb was placed into a trough at the front of the casing, and when the trigger was pulled the spigot rammed into the tail of the bomb and fired it out of the casing and up to approximately 140 metres (150 yd) away. Blacker called the weapon the 'Baby Bombard', and presented it to the War Office in 1941. However, when the weapon was tested it proved to have a host of problems; a War Office report of June 1941 stated that the casing was flimsy and the spigot itself did not always fire when the trigger was pulled, and none of the bombs provided exploded upon contact with the target.
At the time that he developed the Baby Bombard and sent it off the War Office, Blacker was working for a government department known as MD1, which was given the task of developing and delivering weapons for use by guerilla and resistance groups in Occupied Europe. Shortly after the trial of the Baby Bombard, Blacker was posted to other duties, and left the anti-tank weapon in the hands of a colleague in the department, Major Millis Jefferis. Jefferis took the prototype Baby Bombard apart on the floor of his office in MD1 and rebuilt it, and then combined it with a hollow-charge mortar bomb to create what he called the 'Jefferis Shoulder Gun'. Jefferis then had a small number of prototype armour-piercing HEAT rounds made, and took the weapon to be tested at the Small Arms School at Bisley. A Warrant Officer took the Shoulder Gun down to a firing range, aimed it at an armoured target, and pulled the trigger; the Shoulder Gun pierced a hole in the target, but unfortunately also wounded the Warrant Officer when a piece of metal from the exploding round flew back and hit him. Jefferis himself then took the place of the Warrant Officer and fired off several more rounds, all of which pierced the armoured target but without wounding him. Impressed with the weapon, the Ordnance Board of the Small Arms School had the faults with the ammunition corrected, renamed the Shoulder Gun as the Projector, Infantry, Anti Tank, and ordered that it be issued to infantry units as a hand-held anti-tank weapon. Production of the PIAT began at the end of August 1942.
There was disagreement over the name to be given to the new weapon. A press report in 1944 gave credit for both the PIAT and the Blacker Bombard to Jefferis. Blacker took exception to this and suggested to Jefferis that they should divide any award equally after his expenses had been deducted. The Ministry of Supply had already paid Blacker £50,000 for his expenses in relation to the Bombard and PIAT. Churchill himself got involved in the argument; writing to the Secretary of State for war in January 1943 he asked "Why should the name Jefferis shoulder gun be changed to PIAT? Nobody objected to the Boys rifle, although that had a rather odd ring." Churchill supported Jefferis claims, but he did not get his way. For his part Blacker received £25,000 (equivalent to £964,000 in 2015). from the Inventions Board.