United Airlines has made a repeat order for four more Boeing 787-9 aircraft.

The new aircraft are expected to be delivered in 2020.

“These new 787 aircraft are another step in our widebody replacement strategy, and we will continue to roll out new announcements in the future as we implement our comprehensive fleet plan,” said Gerry Laderman, senior vice president of finance and acting chief financial officer of United.

The 787-9 is the longest-range variant of the 787 family and is capable of flying up to 7,635 nautical miles while using 20% less fuel than older-generation airplanes.

The Chicago-based carrier also operates the smaller 787-8 and will be the North American launch customer of the larger 787-10. With this latest purchase, United Airlines has now ordered 55 787s.

“There is no better validation from a customer than a repeat purchase,” said Ihssane Mounir, senior vice president of commercial sales and marketing at Boeing. “We are delighted that a leading carrier such as United Airlines has placed yet another order for the 787 Dreamliner. Their trust and confidence in the 787 speak to the airplane’s unmatched capabilities and superior value. We look forward to delivering more Dreamliners to United for many years to come.”

The 787 is the fastest-selling twin-aisle airplane in history with nearly 1,400 sold and more than 700 delivered to customers, nearly half of whom have placed repeat orders, according to Boeing.