Delta Air Lines said it will order 37 Airbus A321 jetliners, according to a company announcement. The A321 is Airbus’ single-aisle workhorse largely used to domestic flights. These jetliners will help replace Delta’s MD-88 planes, which are aging and not as fuel efficient.

Per Delta Air Lines,

“The Airbus A320 family of aircraft continues to be a cost-efficient, reliable and customer-pleasing mainstay of our narrowbody fleet,” said Ed Bastian, Delta’s incoming chief executive. “The order for the A321s is an opportunistic fleet move that enables us to produce strong returns and cost-effectively accelerate the retirement of Delta’s 116 MD-88s in a capital efficient manner.”

“Delta is an industry leader in many ways, not the least of which is contributing to the trend toward larger, more fuel-efficient aircraft for their single-aisle fleet,” said John Leahy, Airbus Chief Operating Officer – Customers. “The A320 Family continues to be the backbone of every airline in the world that is paying attention to what their passengers want and their investors need. In 2015, nearly 40 percent of our A320 Family deliveries were A321s, up some 10 percent from the previous year. Our customers, like Delta, know where to find the best comfort, economy and reliability.”

The aircraft are set to be delivered through 2019. The additional planes will now cause Delta to have 82 A321 planes. Airbus is based in France. Its competitor is Boeing, which is based in Chicago.