United Airlines recently expanded its Flight Training Center in Denver, Colorado, to nearly half a million square feet, which makes it one of the world’s largest airline training hubs. All of United’s 12,000 pilots come through for initial training and return every nine months to stay up to date on flight procedures and safety protocols. Pilots must complete training in the simulators before flying in real planes. The campus has 31 simulators, each costing $15 million to $20 million. United plans to have a total of 40 simulators in the next 12 to 18 months.

CNBC got an inside look to see what it takes to become a commercial airline pilot.