It has expanded that fleet since, most recently with an announcement on Tuesday that it will increase the number of planes it operates to 70 by 2021. It wants to use these to decrease its Prime delivery time to a maximum of one day.
Amazon will be using the 15 new planes after they were bought by GE Capital Aviation Services (GECAS).
Dave Clark, Senior Vice President of Worldwide Operations at Amazon, said that the new planes "create additional capacity for Amazon Air, building on the investment in our Prime Free One-Day program."
Amazon has been delivering more of its products itself, investing in a vast network of transport systems.
There is also speculation that it is seeking to create a third-party delivery company that other companies could use, bringing it into competition with the likes of UPS.
Converting a plane costs around $5 million for planes that don't require additional maintenance or extra features, Pratt said.
Boeing is ramping up the rate at which it converts 737-800s into BCFs, Pratt said, going from eight planes in 2018 to 17 in 2019.
He said that this conversion process can "add decades" on to the lifespan of a plane.
Boeing is predicting that it will deliver 2,650 freighters between 2018 and 2037, and that more than 60% of these will be converted from passenger planes.