Who invented the U-boats that were used in World War One? How did they come about?

I need to find who invented the U-boats (unterseeboots) that were used and how they came about and how they were improved. It's for a history class. Any help is greatly appreciated.|||The first U-boats were made in 1906. The invention is based on that of William Bourne, but Leonardo da Vinci had the idea earlier.