PARIS—Amazon is adding 15 more Boeing 737-800 freighters to its network following a deal with GE Capital Aviation Services (GECAS), paving the way to expand its fleet to at least 70 aircraft, including 20 GECAS-provided 737s.

“These new aircraft create additional capacity for Amazon Air, building on the investment in our Prime Free One-Day program,” Amazon SVP of worldwide operations Dave Clark said. “By 2021, Amazon Air will have a portfolio of 70 aircraft flying in our dedicated air network.”

Amazon has taken delivery of four 737-800s and the fifth will enter service in July. All are operated by Atlas Air Worldwide subsidiary Southern Air Transport. Amazon has not announced where the next 15 will be placed, but all will fly in the U.S., the company said. Boeing is converting the aircraft into freighters for GECAS.

Beyond the five 737-800s, Amazon’s fleet will include 50 767s by the end of 2019. The fleet’s operation is split between Atlas and Air Transport Services Group.

The carrier has been operating the 737s on thinner routes, and plans to continue to use them to right-size capacity. Amazon will open new air facilities this year at Fort Worth Alliance Airport, Wilmington Air Park, and Chicago Rockford International Airport. Its main hub at the Cincinnati/Northern Kentucky International Airport will open in 2021.