At the beginning of World War I there were some armored cars in use. Basically they were car or truck chassis that had armor and weapons added. Often this would greatly overweigh the vehicle and cause many breakdowns. Armored cars were good for reconnaissance but once the war settled down into a slugging match in the trenches they weren't as useful on the Western Front. On the Eastern Front they were capable of being much more mobile.
In 1912 Gunther Bursztyn, an Austrian officer, had proposed a vehicle that would use tracks and a main gun in a turret. Lancelot de Mole put forth a similar design to the British military and Captain Levavasseur to the French military but all three were not developed as they weren't seen as being needed by their respective governments.
As World War I progressed it was realized that a weapon that could go across shell covered ground, protect its crew, and destroy enemy strong points was needed.
A Landship Committee was formed and the British Navy lead the effort as they had more experience with armored cars.
Little Willie was designed and had a 2 pdr gun and a machine gun for armament. Wheels at the rear of the tank was the primary method of steering. The tank could climb a 1' / 0.3m obstacle and cross a trench 5' / 1.5 m wide.
The Committee decided that a more advanced tank was needed and Mother was developed. This ended up being the design that was choosen. It had a rhomboidal shape that could cross a trench 9' / 2.7 m wide. Four of the crew was needed to drive Mother. There was no suspension and this lead to a very rough ride and even injuries to the crew while going across very rough terrain. Mother was propelled by a 105 HP engine.
100 were ordered for production and it became the Mk I. One of its main problems was that it broke down a lot.
Weapons were placed in sponsons on the side allowing for firing into enemy trenches.
The development was cloaked in secrecy by making up a story that they were making mobile water cisterns (tanks) for use on the Eastern Front and the boxes were even labeled "with care to Petrograd" in the Cyrillic alphabet.
The first true tank with an all-around traverse was the French FT-17 first introduced in 1917. It had a short 37 mm gun or a single MG as it's main armament. It had 22 mm of armor and a maximum speed of 5 mph. There was a crew of 2.
The layout of the first tanks consisted of the front compartment with the driver, a fighting compartment with the turret and gun, and the engine compartment with the engine and fuel.
Initially it was thought that the machine gun would be sufficient as a tank's main armament but it was soon realized that tanks would meet tanks on a battlefield and more powerful guns would be needed to fight the enemy. The guns needed to be powerful enough to penetrate the thickest armor on opposing tanks.
Thus the tank needed to have a large enough turret ring diameter to be able to absorb the recoil of the main gun, store ammunition, and hopefully enough room to allow fitting a larger weapon at a later time. This diameter would then determine the width of the hull. For the British, the designers were limited by the standard railway gauge and the German's were restricted to a 24-ton bridge-load specification.
Tanks were usually designed with machine guns as secondary armament. Often a machine gun was installed coaxially with the main gun. Also a machine gun was sometimes installed in the hull front for use by the driver or co-driver.
The main gun was placed in a thick piece of armor known as a mantlet that went across the front of the turret. The turret contained all the necessary hardware to aim, load, and fire the gun. The British and Germans usually had 3-man crews in the turret. The commander sat in the rear, the loader on one side, and the gunner on the other side of the gun. In British tanks the radio was located in the turret and the loader was its operator. The American, French, and German tanks had the radios placed in the hull and were often operated by the co-driver
Engines were selected that would move at a required speed. The American's selected rotary aircraft engines and this resulted in the tanks having a high silhouette. The British would use flatter engines and most of their tanks were lower. The Japanese developed an excellent air cooled diesel engine for use in the tropical climate.
From the engine the drive shaft passed through the gearbox to the drive sprockets to drive the tracks. Most gearboxes were synchromesh or "crash." The synchromesh required the driver to double de-clutch. The tracks acted as a brake during the gear changes.
There were several different types of suspensions that a tank could have. Coil springs that attached to pivoting bogie wheels were the most common. Torsion bar had the road wheels suspended from arms attached to tension-sprung transverse bars (introduced by Germans). The Christie suspension was developed by American Walter Christie. This consisted of large road wheels that hung from cranks that had vertical coil springs housed in the hull. This was used by the Russians in their BT series and the British their Cruiser tanks.
The tracks were made from cast iron at first, and replaced by manganese steel. The American's developed tracks from rubber blocks and linked by metal.
Most tanks were designed with armor that was primarily flat. In 1937 the Russians designed the precursor to the T-34, the A-20 which used sloping armor. This principle was that a 45 mm thick put at an angle of 60 degrees had the equivalent of 90 mm of armor.
Early tanks had bolted on armor which could be dangerous if a shell struck the head of the bolt as the inside part could break off and fly around the inside of the vehicle and possibly injure the crew. The Germans and Russians developed joint welding in the mid-1930s and this helped reduce the weight of the vehicles as well as helping to increase production.
During World War I when tanks were first used there was little that senior commanders could do other than give them an objective and let them go. To assist the tanks the commanders would be on foot, while under fire, running between vehicles to guide them on what to do.
In the 1920s hand flags were used by some armies and some (primarily Russia) used them throughout World War II. However, this was less than ideal as the commander had to be partially exposed and as soon as they were used the enemy would most likely immediately target the commander's tank.
Radios were available during World War I but they had little range and were bulky. It wasn't until 1931 when the 1 Brigade of the British Royal Tank Corps used voice transmissions to control the movements of 180 tanks during the Salisbury Plain exercises. Soon the other major powers, except for Russia, Japan, and Italy, started installing radios into their vehicles.
Some in military circles didn't have much hope for tanks after World War I, but there were some who were visionaries.
Colonel J.F.C. Fuller wrote in his Plan 1919 that he saw a main body holding down the enemies forces while tanks and truck born infantry would deeply penetrate and destroy the enemies headquarters. Someone who shared his views was Captain Basil Liddell Hart, the military correspondent of the Daily Telegraph.
Many in the higher echelons of the military needed "real proof" about these theories. Some did see the tank as supporting the infantry and some saw it in the role as calvary.