Leonardo's crossbow is a type of shooting weapon designed by Leonardo da Vinci, whose drawings are in the Codex Atlanticus. Never constructed by its designer, it was instead built to a scale of 1:1, as shown in the ITN documentary Leonardo's Dream Machines, which was aired for the first time in February 2003 by Channel 4. The original idea of Leonardo, as described in the drawings of the Atlantic Codex (1488–1489), was to build a giant crossbow in order to increase the range of the dart. It was used to fire rocks and bombs, it was mostly intimidation based. The bow was made up of thin wood, on 6 wheels, 27 yards across, and made up of 39 separate parts.
The creation of the design is linked to Ludovico Sforza, an Italian prince in the Renaissance era, who wanted to expand and advance both his military and the Milan region. To do so, he wanted to update the current treatise on military engineering by Roberto Valturio. Leonardo responded by writing Sforza a letter that included a number of innovative machine designs, one of which was the crossbow. Leonardo also highlighted in the letter his expertise in engineering, most likely having known that Sforza was wanting to hire military engineers at the time. 
While there is no exact date for the illustration of Leonardo's crossbow, it is generally believed that it was completed between 1483 and the early 1490s.  Many scholars generally agree that Leonardo completed the drawing in Milan, but there is debate as to why he came there. Some believe he came to Milan in search of work as a painter and then he got news of Sforza's military desires upon arriving; others suggest that the initial reason he came to Milan was to work for Sforza. 
While some believe that Leonardo designed the crossbow for his own amusement, the context around this design suggests the crossbow was intended to be a dangerous weapon that would greatly appeal to his employer: Ludovico Sforza.  Inspiration for such a weapon most likely stemmed from the fact that Leonardo grew up in Italy during the 15th century, which means he was a witness to the constant warfare between the many city-states in his area. Thus, Leonardo put extensive time and effort into designs that could both protect his fellow citizens and greatly harm the enemy. The crossbow supports this theory in that the intended enormous size of the weapon was meant to invoke fear and panic in its enemies to keep them away, but it would still have the capability to cause great damage and injury had it been constructed. 
The crossbow had been around long before Leonardo's design, but his design made the weapon more advanced, which is why Leonardo's crossbow stands out as significant. If a crossbow is designed with a narrower shaft and a tapered bolt, which adjusts the nocking of arrows, it greatly improves the airflow of the bow and the drag on arrows; this allows the crossbow to operate much more efficiently and have a more precise aim. These ideas were present in Leonardo's design and originated with Leonardo alone. 
The mathematics that Leonardo utilized to construct his crossbow design were far advanced despite having some now known inaccuracies with today's current knowledge of geometrics and design. Nonetheless, Leonardo was "the first modern engineer to attempt to apply the geometrical mathematics of the laws of motion to the design of machines."  The other mathematical marvel that is noted in Leonardo's design of the crossbow is the proportional techniques that he utilized in every aspect of the design. 
- (in Italian) Balestra gigante
- IMDb, Leonardo's Dream Machines
- Landrus, Matthew (2010). Leonardo da Vinci's Giant Crossbow. Berlin, Heidelberg: Springer. p. 5,17,33.
- Landrus, Matthew (2010). Leonardo da Vinci's Giant Crossbow. Berlin, Heidelberg: Springer. p. 5.
- Landrus, Matthew (2010). Leonardo da Vinci's Giant Crossbow. Berlin, Heidelberg: Springer. p. 9,21.
- Stuart, Nathan (2016). "The Engineering Genius of a Renaissance Man". The Engineer. Online.
- Foley, Vernard; Palmer, George; Werner, Soedel (1985). "The Crossbow". Scientific American. 252 (1): 104-111.
- Landrus, Matthew (2010). Leonardo da Vinci's Giant Crossbow. Berlin, Heidelberg: Springer. p. VIII.
- Landrus, Matthew (2010). Leonardo da Vinci's Giant Crossbow. Berlin, Heidelberg: Springer. p. 3.
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