Prior to World War II, the United States Army — and many other armies — simply thought of tanks as having one purpose: to support the infantry. They really weren't intended to deal with other tanks; a different vehicle had that job.
Those vehicles were known, aptly, as tank destroyers. The Achilles, which served with the British Army in World War II, was one such tank destroyer. This vehicle was a modified version of the American M10 Wolverine. The big difference between the two vehicles was that the American version had a three-inch gun, while the British Achilles had a quick-firing (or QF) 17-pounder gun (which referred to the weight of the shell fired).
Primary armaments aside, the two vehicles were a lot alike. They had top speeds of 25 miles per hour, could go 200 miles on a tank of fuel, packed a M2 .50-caliber machine gun as a secondary weapon, and had a crew of five. The Wolverine held 54 rounds, while the Achilles had 50.
The 17-pounder gun of the Achilles was able to kill just about any Axis tank in a fight. The tank destroyer was intended to move fast and hit enemy tanks hard, using their speed to get into a good position to hit the enemy tanks and then scoot from the firing position. This was a good thing for the Wolverine and Achilles since there was one thing they lacked: armor.
The real problem, though, was that, all too often, the tanks supporting the infantry, like the M4 Sherman, ended up fighting the German tanks. Eventually, Shermans received tank-busting guns, like the 17-pounder and the three-inch gun.
Today, the tank destroyer concept centers around faster, lightly-armed vehicles that carry anti-tank missiles, as opposed to guns. Today's tank is still seen as an infantry-support vehicle, but it is also capable of killing enemy tanks.
Watch this video from the Smithsonian Channel to learn more about the Achilles.