Amazon promises speedy delivery on millions of items, and launched its own airline in 2016 so it could have its own cargo planes to fulfill that promise.
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).
These new cargo planes are actually converted Boeing 737-800 passenger planes, which Boeing calls "BCF"s, or Boeing Converted Freighters. You can see the passenger windows blocked out on the side of the plane.
But Boeing also created a whole new door on the side of the plane to install cargo, which is hydraulically operated.
Crew enter the plane by the smaller side door, where they enter a space between the cockpit and the cargo hold.
The cargo hold is separated from the crew area, but is accessible through a small door. The wall protects the crew from the force of the cargo, in case it moves.
The ground is reinforced, and more cargo is stored underneath. It can carry almost 23 tons of freight.
Only two people can enter the area at a time during a flight to handle cargo, including animals and perishable goods like food.
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.
This plane originally entered service in 2004, and was recently converted. Alvey Pratt, the director for converted freighters at Boeing Global Services, said the conversion process is "relatively straightforward" and takes around 90 days.
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.