What vehicle did Leonardo da Vinci design as an early prototype of the modern tank?

Leonardo da Vinci designed an armored fighting vehicle that is often described as an early prototype of the modern tank.

The vehicle was a conical, turtle-like structure covered with armor plates. Inside, people would turn cranks connected to gears, driving several wheels and allowing the machine to move across the battlefield.

Leonardo proposed the design while working for Ludovico Sforza, the Duke of Milan, in the late fifteenth century. The vehicle was intended to intimidate opposing troops and provide protection for the people operating it.

It was never built for warfare. A famous modern observation is that the surviving drawing appears to contain a mechanical flaw: the gears may have been arranged so the wheels would move in opposite directions. Some historians think this may have been deliberate or simply a drawing error.