The latest push to speed delivery of its products comes as the company ships an increasing number of packages worldwide.
Amazon's parcel volume was an estimated 1 billion packages in 2015 the same number that FedEx delivered three years earlier for hundreds of thousands of customers.
Amazon has had issues with the reliability of air freight services. In 2013, it offered refunds to customers who got their Christmas orders late after bad weather and a jump in online shopping caused delays for UPS and FedEx.
"They're such a big online retailer," said Satish Jindel, president of shipping consultant ShipMatrix. "There's so much volume that if you have to add transportation for yourself, why would you pay a retail price when you can get wholesale? It makes sense."
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The remaining freighters will be rolled out in the next couple of years. Aircraft like Amazon One allow the company to "continue to maintain our fast delivery speeds and lower our costs as our Prime base and our Prime member growth continue to soar," said Dave Clark, Amazon's senior vice president of worldwide operations.
Despite its growing fleet of aircraft, Amazon said it plans to continue to use FedEx, UPS and other transportation partners.
The company has been furiously building out distribution centers, where workers and robots pull products off shelves and package them for delivery, as well as smaller sorting plants, which arrange packages by ZIP code for faster delivery. It has a network of more than 125 fulfillment centers worldwide.