Depending on who you are, the history of slots can start in a number of different ways. There are many historical records around of ancient cultures that used to have a slot machine system that worked in the same way, except instead of the electronic slot machines that we are accustomed to, it was actually a concentration type game in which the contestant picked cards from a face down pile and their prize was awarded to them based on the combination of cards that they picked. Such games are still available today in carnivals and many people would argue that these types of games were the basis for modern slot machines.
As for modern slot machines however, most people would argue that the modern slot machine is purely a Las Vegas invention. While there were similar games involving cards available in the casinos of Europe, the modern slot machine would not really be possible without the advent of electricity and that did not happen until the 20th century. Most of the modern slot machines that are popular in the world today were invented in Las Vegas and for that reason it is quite appropriate to credit Sin City with being the inventor of the modern slot machine.
With online slot machines and the free slots that came along with them however, it is much harder to make that type of value judgment. It is certainly true that there are many online casinos within the United States initially that came up with the code necessary to create such online slot machines, but the simple fact of the matter is that most of the online slot machines of the modern day were created outside of the country in places like South America and Asia. For this reason, it is quite possible that the history of slots is now shifting from a base of North America and Europe to a base of South America and Asia.