Amazon's billionaire founder Jeff Bezos said on Monday he and his brother Mark would fly on the first crewed space flight from his rocket company, Blue Origin.

"Ever since I was five years old, I've dreamed of travelling to space. On July 20, I will take that journey with my brother," Bezos said in an Instagram post.

Bezos, who will step down as Amazon's chief executive on July 5, will join the winner of an auction for a seat on the first space flight from Blue Origin in the same month.

Bezos and fellow billionaire Elon Musk have been investing heavily on their rocket startups, but Blue Origin and Musk's SpaceX have so far only sent satellites for clients into orbit.

Blue Origin closed the first round of the auction last month and said it had received more than 5,200 bidders from 136 countries, without disclosing the highest bid from the round.

The current highest bid stood at $2.8 million in the ongoing second round of auction, according to Blue Origin's website.

Blue Origin is targeting July 20 for its first suborbital sightseeing trip on its spacecraft, a landmark moment in a competition to usher in a new era of private commercial space travel.

The New Shepard rocket-and-capsule combo is designed to autonomously fly six passengers more than 100 kilometres above Earth into suborbital space, high enough to experience a few minutes of weightlessness and see the curvature of the planet before the pressurised capsule returns to earth under parachutes.

The capsule features six observation windows that Blue Origin says are nearly three times as tall as those on a Boeing 747 jetliner and the biggest ever used in space.

Reuters reported in 2018 that Blue Origin was planning to charge passengers at least $200,000 for the ride, based on an appraisal of rival plans from billionaire Richard Branson's Virgin Galactic Holdings Inc and other considerations, though its thinking may have changed.