The race between Boeing and SpaceX was born in the years after the space shuttle made its final flight.
Without the shuttle, which was retired in 2011, the United States had no way to launch NASA astronauts to the International Space Station, instead relying on Russian Soyuz capsules to take Americans into orbit.
NASA needed a new ride, and in 2014 the agency announced two companies would compete to be the first to again fly Americans to space: Boeing and SpaceX.
"Boeing and SpaceX have each presented to us designs that will allow us to fly crews to the International Space Station within a few years," NASA Administrator Charlie Bolden said at the time.
The two companies could not have been more different: Boeing, the legacy aerospace firm with more than a century of experience, and the relative newcomer SpaceX founded by billionaire Elon Musk.
Boeing received $4.2 billion from NASA to build its Starliner ship, while SpaceX got only a little more than half of that to develop its Dragon crew capsule.
In the end, it was a race SpaceX would win, launching two astronauts from Florida to the space station in the summer of 2020.
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"This event is something that all of humanity can get excited about," Musk said after the launch in 2020.
In the meantime, Boeing has struggled with Starliner. An uncrewed December 2019 test flight was fraught with technical problems that prevented the craft from reaching the space station.
It was a costly error that required Boeing to launch a second uncrewed test flight in 2022.
Boeing has yet to fly Starliner with astronauts on board. It will try again on Saturday after pushing back recent launch dates due to a faulty valve and a helium leak.
NASA veterans Barry Wilmore and Sunita Williams will be aboard. It will be Starliner's first human mission to the space station, compared to SpaceX, which has already flown eight.