More people are using Amazon Prime than ever. The online retail giant surpassed 100 million Prime subscribers earlier this year, and Cyber Monday 2018 was the site's biggest shopping day yet. So how will Amazon keep up with this demand? Apparently by buying more airplanes.

On Friday, Amazon announced that it was expanding its fleet to 50 aircraft (up from 40). Amazon says this is to support the increasing number of Prime subscribers who expect free two-day delivery. (On top of that, Amazon customers bought 2 billion items with one-day delivery this year.)

By adding 10 more aircraft, Amazon is expanding its fleet by 25 percent -- a sizable increase. But this fleet is still dwarfed by other shipping companies. UPS owns 247 aircraft (and leases more during November and December), while FedEx owns over 650 (ranking it among the world's largest airlines).

If Amazon is to continue offering free-two day shipping to its millions of Prime subscribers, it better have the arsenal to back it up.