Amazon on Tuesday said it agreed to lease 15 more Boeing cargo planes from GE Capital Aviation Services, helping the e-commerce titan continue growing its air fleet so it can speed up deliveries.

The 15 Boeing 737-800 planes are in addition to the five planes Amazon already agreed to lease from GECAS earlier this year.

The company has 42 planes flying today, and by 2021 it will operate 70 planes.

Amazon expanded its fleet as part of its expensive effort to transition its Prime two-day shipping program to one day in the US.

That work is already resulting in packages arriving earlier at Prime customers' doorsteps and may help Amazon maintain its competitive edge against rivals like Walmart.

Amazon said in April it was spending $800 million this quarter to bolster its shipping infrastructure to support the change.

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