The first Nautilus was an American design, and was built in France, but it was not Verne's inspiration. Robert Fulton's Nautilus of 1801 was a project based on ( some might say, stolen from) David Bushnell and his Turtle of 1775. Fulton offered his submarine to both Napoleon and the British (and tried to tempt the Russians as well) but his project was shelved, denied funds, after he'd been drawn to come to England ( the British had a very good navy, and to them the major benefit of Fulton's submarine was achieved by denying it to everyone else). Fulton was able, however, to extract from the British the rights to export a steamboat engine to the US, which would power his very successful North River Steamboat in 1809.

Though Verne and Fulton had the same name on their boats, what inspired Verne was the Catalan inventor Narcis Monturiol. Monturiol built a small submarine, the Ictineo, in 1859. He intended it to be used for underwater exploration. He was able to make several dives with it. But like all human-powered submarines, it went very, very slowly: even a rather small submarine displaces a lot of water, and that means it has to be quite heavy and has a lot of inertia. The Ictineo was 10 tons: even though water has little friction at low speeds, imagine trying to start, push, and stop 10 tons with a pair of oars, and you'll get an idea of the difficulty.

After Ictineo was damaged by another ship, Monturiol was able to form a company and with a very popular public appeal raised considerable amounts of money for another sub. That, the Ictineo II, was larger, built with a metal hull, and was also fit up with a steam engine. Monturiol tried to develop a chemical heat source that could run the engine without needing oxygen, and so run underwater. He did have some success with running the boat on the surface with regular combustion, but was unable to successfully develop his sub marine into something that could be powered while submerged ( and also make money ) before his company went bankrupt in 1867. Whether his engine could have ever gotten to work at that time can be argued , but there would not be a successful anaerobic submarine engine until the Germans made one in 1940.

Though Monturiol's company went bankrupt, the project created a great deal of publicity and interest. His bankruptcy was in December, 1867: Verne would publish the first installment of his serialized 20,000 Leagues Under the Sea in March of 1869.

Puig-Pla, C., & Roca-Rosell, A. (2003). A SPANISH PROJECT FOR SUBMARINE NAVIGATION: NARCÍS MONTURIOL AND THE STRUGGLE FOR DEMOCRACY. Icon, 9, 128–143. http://www.jstor.org/stable/23790674