Welcome to the easy-to-understand catapult history lesson. I’ve tried to keep this simple and clear. You will find clickable, superscript numbers in places where I wanted to link to my source. I have researched catapult history over and over. If you do find an error or inconsistency, please let me know in the Contact Us page. You won’t find any.
When was the Catapult Invented
If you consider that a catapult is any device that throws something, then the first catapult was invented when Ogg the Caveman first flung a stone using a stick. However, we are really only interested in the big, siege-engine catapults like the ballista, the onager and the trebuchet when we examine catapult history.
Catapult: A class of non-handheld devices designed to hurl a projectile a great distance by a means of force other than combustion.MyWizards.com
Even when limiting it to this definition, though, the research is still tricky.
Shaduf – The Accidental Catapult
The Egyptians used a device called the Shaduf to move water in 1500 B.C.E.  While the Shaduf wasn’t designed to be a catapult, it did use a counterweight to quickly lift a heavy bucket of water. There are many elements here that come close to satisfying the definition of catapult. For example, it is a lever that uses a human powered counterweight. It even has a sling AND a bucket. So, who’s to say that someone never discovered that if you pulled the weight down too quickly, it would send the bucket across the field?
Unfortunately, the shaduf does not meet all of the requirements of a catapult. Specifically, it was not designed with the intent of hurling stuff. So, while it may be a predecessor to the trebuchet, it isn’t a catapult. It is still pretty cool, though.
Beginnings of Catapult History
In the beginning there was a rock and a stick. Ogg the Caveman would throw the rock and/or the stick at the caveman next door. This was not a catapult. This was just mean. Weapons improved, the stick became an arrow and the rock just stayed a regular old rock. But you could hurl that rock at someone far away if you put it in a sling.
The bow allowed us to shoot a stick farther than our enemy could throw one back at us. This was called the advantage of range. If I can shoot you before you get close enough to shoot me, I win. Soon, both sides had bows. The advantage of range then went to whoever had the bigger, better bow because they could shoot farther. So everyone started trying to build better bows.
And thus, the bow begat the longbow, which begat the recurve bow, which begat the composite bow , which begat the catapult, which begat the crossbow.
Actual Catapult History
Well, it was an attempt to make a bigger, better bow that could shoot further than the bows of the enemy. Someone took a really large composite bow, one much too big for a person to use, and stuck it, sidewise on a board. They called it the gastraphetes (belly bow). It was too big for a person to hold and fire comfortably, so it would have needed to be propped up on a wall or mounted on legs. 
With the bow at full draw, the weapon could be lifted into firing position. This must usually have involved propping it on a wall, given the weight and bulk of the machine; otherwise, the archer would have required a portable prop…Duncan B. Campbell
Greek and Roman Artillery 399 BC – AD 363
So what do we know?
- The gastraphetes was a device too big to be hand-held.
- It was designed to fire an arrow a great distance.
- It used a composite bow for power and not combustibles.
Eureka! The gastraphetes was a catapult. Possibly the first catapult, but certainly the first recorded catapult class device.
When was the Gastraphetes Invented
The gastraphetes was invented around 420 BC.   If you look around on the web for this answer you will find a few different ones. Sometimes, the invention of the catapult is set at 399 BC, but this is wrong. This misinformation came about because a man named E.W. Marsden misinterpreted the writings of a Greek author named Diodorus Siculus. It has since been determined that the really heavy gastraphetes was invented twenty-one years before that, in 420 BC. Don’t be too hard on Marsden, either. He got most everything else right.
We don’t know who invented the gastraphetes, but we know that Zopyrus of Tarentum was an engineer that made some improvements to it. While he is not the inventor, his is the earliest name we have for any engineering work done on the weapon.
Catapult vs Crossbow
It interesting to note that the catapult was invented to make a bigger bow. The crossbow was then invented to try to make a smaller catapult. I had always assumed the crossbow came first.
The Ballista Catapult
Next to show up was the ballista. This is a large gastraphetes that uses torsion instead of tension as a power source. It was the engineers of Dionysius in 399 BC that created the first torsion-powered, arrow-firing catapult. We now call this weapon the ballista. Torsion power was a change from the previous arrow-firing catapults that were powered by the composite bow.
This weapon dominated the land for the next two hundred years.