A masked figure stands, spotlit, in the gloom of the Science Museum’s basement, looking part nuclear radiation agent, part Star Wars desert bandit. A domed leather hood covers its face, punctured by square blue goggles and a breathing tube sprouting from the nose, while a pair of hessian sacks dangle from its arms.

This eery costume is a 15th-century diving suit, drawn by Renaissance polymath Leonardo Da Vinci, designed not for studying the secrets of the sea or the anatomy of fish, but for sabotaging enemy ships. Nearby stands a U-shaped metal clamp fitted with a threatening screw – another of Leonardo’s inventions, with which divers could smash open the wooden hulls of their opponents’ boats.

It is one of a host of wily devices on show in Leonardo da Vinci: The Mechanics of Genius, which aims to bring his sketchbook doodles to life and shine a light on the artist-inventor’s technological cunning. Among the 39 contraptions on display are machines for cutting files, boring holes, digging canals and weaving cloth, and a dreamy section dedicated to his fanciful visions for flying machines, from flapping birds’ wings to a spiralling “aerial screw”. What stands out is the sheer amount of time the famous painter of angelic faces spent coming up with ever more elaborate instruments for death and destruction.

One of the first items in this show is a gargantuan crossbow, standing upright like a sinister crucifix, designed to propel flaming balls at enemy ranks. The model is alarming, but the real thing would have been the height of a three-storey house, its bow stretching 25 metres across. There’s no evidence it was ever built, but it reveals Leonardo’s understanding of the psychology of war: that the fear a menacing contraption could instil was as important as the damage it could inflict.

He deployed the same approach in a ferocious triple-loaded cannon, with three banks of 11 guns mounted on a rotating axle, so that one set could be reloaded while the others were firing, for maximum bombardment. It would have been impossibly cumbersome to wheel around, but it certainly looks the part. As does the UFO-shaped armoured vehicle, designed to be propelled by eight men turning cranks inside its metal shell, along with a fearsome chariot adorned with rotating scythes to slice through enemy soldiers – and chop off its own horses’ legs in the process, by the look of it.

“We’re not sure if many of his inventions would have actually worked very well,” says Sue Mossman, project leader at the Science Museum. “Scholars have speculated that he intentionally built faults into his weapon designs because he was known to be a pacifist, while others think he never intended them to be built.”

This uncertainty made the production of these models (first built in Milan in 1952 for a show celebrating the 500th anniversary of Leonardo’s birth) a tricky process, requiring a good deal of artistic interpretation. Still, wherever possible, his mechanisms are brought to life in hands-on Science Museum style with simplified interactive models, so you can turn a wheel and watch the cogs whir.

Whether they were to be built or not, Leonardo’s sketches for ferocious instruments of war proved a handy calling card. In bellicose 15th-century Italy, where rival dukes and princes waged constant battle, posing as an itinerant weapons salesman was a clever way to befriend your next patron, who might commission a nice fresco along with the dagger-studded siege engine.

In a letter to Ludovico il Moro, Duke of Milan, we see Leonardo trying to woo the nobleman in an attempt to enter the service of the ruling Sforza family. “I have also kinds of cannon most commodious and easy to carry,” he writes, “with which to throw inflammable materials, whose smoke causes great fright to the enemy, with serious injury and confusion.”

Even if most of Leonardo’s more bloodthirsty contraptions remained on the drawing board, they fertilised his voracious forays into other fields of inquiry. As his mind wandered from how to scale a wall to how to spin silk thread, he was always seeking to automate manual processes or improve on the designs of fellow engineers. His intense study of crossbow mechanics, for example, led to a proposal for a self-propelled vehicle, driven by a complex matrix of pre-tensioned crossbows, springs and toothed wheels.

This is the elephant in the room, when the exhibition moves on to contemporary inventions inspired by his designs. Five hundred years on, scientific breakthroughs are still fostered by the military industry, innovation forever fuelled by man’s ravenous appetite for destruction.

• Leonardo da Vinci: The Mechanics of Genius is at the Science Museum, London, from 10 February to 4 September.