Amazon has revealed it recently started designing an eighth-generation Prime Air delivery drone.

In a letter to shareholders published via the US Securities and Exchange Commission, Jeff Bezos, chief executive officer at Amazon, announced that the Prime Air team is "in the design phase on generations seven and eight". It is even flight testing fifth- and sixth-generation aerial vehicles.

The letter doesn't reveal any specifics about the latest Prime Air delivery drones, but Bezos said in December that 86 per cent of Amazon's packages are under 5 pounds, and that the drones could carry them up to 10 miles from an Amazon fulfillment center.

Although it might seem like Amazon is working quickly, keep in mind that Bezos also once said the company was four or five years away from a public launch. After all, Amazon has to work with the US Federal Aviation Administration to make its Prime Air program both legal and safe.

READ:US FAA okays use of most electronics during take-off

Speaking of the FAA, Bezos took credit for the agency's decision to permit in-flight electronics. He said Amazon's public policy team worked with many allies over a four-year period to convince the FAA. They even loaded a test plane with 150 active Kindles: "Yes, it all worked fine," Bezos added.

And finally, Bezos said Prime Instant Video has experienced "tremendous growth", which includes new customers, repeat usage, and streams, and that there were "tens of millions of members" worldwide. Oh, and there are now more than 200,000 apps for Kindle available in 200 countries.