From the moment at Agincourt when a single soldier was killed by a gunpowder weapon, armour protection became more and more impractical. The large scale of issue needed and the cost due to lack of manufacturing meant for most of history the best protection a soldier could wish for was a padded outfit.
This carried on into the first industrial war, World War One. In the early years of that war cloth caps were the norm, however with modern warfare modern mass production arrived and the benefits of metal helmets were realised and issued en-masse.
In the First World War several attempts at protecting the soldier from enemy fire were also tried, the best known was the German Lobster armour. However there were several other concepts tried.
One was the ill-conceived MacAdam Shovel from Canada. It was designed, in 1913, to function as not only a shovel, but also as a bullet shield. The idea being that the soldier stuck the blade of the shovel into the ground, this gave him an armoured screen with a loop hole in it so he could fire at the enemy, while their return fire was deflected by the blade.
MacAdam shovel in action, notice how the loophole is actually to low to the ground to be used without the mound of earth.
In reality the shovels never made it closer to the front than England and were finally sold off as fifty tons of scrap at a massive loss to the Canadian Government.
In-between the wars one British officer, Giffard Martel, came up with an idea for a one man tank. The idea is said to have come from a discussion with another British officer who witnessed a French tank attack during the First World War, and the swarm of FT-17's. When Martel pitched his idea it was laughed at, so he cannibalised an old car and rebuilt it at home in the shape of a one man tank. His plan was for each infantry unit to have a handful of these, and they would advance with the infantry giving covering fire. When the difficulties of one man doing all the jobs for the tank arose Martel countered by pointing to fighter pilots doing all the jobs required in their one man vehicles. Martel went one step further and proposed that every infantryman should be mounted in one of these tankettes.
Martel's one man tank. this is the MK2 version, made out of actual metal. the Prototype had been made of wood.
The basic problem was that technology, at the time, couldn't provide armour thick enough to provide protection to prevent high velocity projectiles from penetrating the armour and causing damage. There were also certain places where low velocity projectiles would cause fatalities. Adding to the issue was that the Army had imposed a weight limit. The armour was designed to cover the most amount of vulnerable body parts within the constraints of the weight limit.
The armour was mostly used by the RAF and 21st Army Group. There's anecdotal reports of it going ashore on D-Day and fighting through Normandy. It was definitely used during Operation Market Garden.
My thanks to Volketten from the NA server, whom is a real expert in these matters and helped with this article.