FedEx is buying six Boeing 767 freighters valued at $1.2 billion as the world’s biggest cargo airline continues to modernize its fleet.

The six aircraft are scheduled for delivery in fiscal 2019 and 2020, according to a FedEx spokesman.

They were among the options included in a 50-jet order that FedEx placed almost a year ago to help reduce operating costs and improve fuel efficiency. Discounts from list value are typical in the aerospace industry.

The wide-body 767, developed in the 1980s, has enjoyed a resurgence as a freighter after passenger airlines switched to aircraft such as Boeing Co.’s 787 Dreamliner and Airbus Group SE’s A330.

The U.S. Air Force has committed to a tanker based on the 767 that could lead to an order of as many as 400. Amazon.com Inc. also is relying on the jet for its nascent air-cargo service.

Boeing currently makes two of the planes a month as it attempts to resolve development snarls with the tanker.

It plans to speed output to 2.5 jets a month next year to accommodate FedEx’s 2015 order, with deliveries running over a five-year period beginning in the fiscal year that runs through May 2018.

Amazon this year announced two agreements to lease a total of 40 767-300s from Air Transport Services Group Inc. for a fledging network that would lessen the Internet retailer’s dependence for deliveries on FedEx and United Parcel Service Inc.

As part of an agreement announced in May, Amazon could take as much as a 30 percent stake in Air Transport Services.