A Hollywood film producer is building the largest mansion in US history - and he wants to sell it for $500 million

Will Smith may be the "Prince of Bel-Air," but Nile Niami is the king.

The film producer and residential developer is in the process of building the largest home in US history on a hilltop in Southern California.

His selling price: $500 million.

The kingdom won't be completed for another 20 months, so start saving.