Boeing CEO Dennis Muilenburg believes a growing shortage of pilots represents "one of the biggest challenges" facing the airline industry.

His comments come at a time when more people are flying to more places, but the number of pilots being trained are unable to keep up with demand.

Demand for air travel is growing so rapidly that 800,000 new pilots are expected to be needed over the next 20 years, according to Boeing's latest forecast.

The biggest need is in the Asia-Pacific region, where an improving economy in China has resulted in more people booking flights. More people are flying in the U.S. too but, at the same time, experienced pilots are reaching the mandatory retirement age of 65 years old.

Speaking to CNBC's Phil LeBeau at the Paris Air Show on Monday, Muilenburg described a global pilot shortage as "one of the biggest challenges we have going forward."

Muilenburg said that, according to Boeing's latest outlook, the market place would climb to $8.7 trillion, up from $8.1 trillion, over the next 10 years. He also estimated the number of new commercial airplanes would rise to 44,000, up from 43,000, over the next two decades.