Boeing 737, the world's best-selling airplane, just reached another impressive milestone. On Thursday, Boeing delivered the 9,000th 737 to China United Airlines, a low-cost subsidiary of China Eastern Airlines.

The U.S. manufacturer and China United held a ceremony to celebrate the milestone delivery at Boeing Field.

From the Classics, to the Next-Generation, to the MAX -  the 737 just keeps getting better. With one 737 taking off or landing every 1.7 seconds, the airplane is the industry's workhorse. On average, over 2,400 737s are in the air at any given time.