These are the 21 richest American entrepreneurs under 40 - and they all made their fortunes in tech

Forbes has released its first-ever list of the richest American entrepreneurs under 40 years old, and it's completely dominated by the tech industry in California.

Of the 40 entrepreneurs they looked at, a full 34 made their money in tech, with 33 living in California. They included those who were either born in the US or who built their companies here.

When we narrowed the focus to only those who had made over $1 billion, the list dropped to 21, and every single one had made their fortune in tech.

These mostly self-made billionaires include founders of some of the most iconic tech companies of the past few years, like Airbnb, Facebook, Twitter, and Uber.

Here are the 21 youngest billionaires in the country, who prove that tech is now ground zero for the American Dream.

Additional reporting by Tess Danielson and Tanza Loudenback.