When test flights do launch, Boeing's flight is dubbed Orbital Flight Test followed by Crew Flight Test. SpaceX's first flight is dubbed Demonstration Mission 1 followed by Demonstration Mission 2.
Veterans of two spaceflights each, Behnken and Hurley will lift off atop a SpaceX Falcon 9 rocket from the Kennedy Space Center's Pad 39A - the same Florida launch pad where the space shuttle left Earth for the last time in July 2011 with Hurley as pilot.
The first Boeing Starliner mission to the ISS will be crewed by John Cassada, a NASA astronaut, physicist and Navy test pilot. Since the retirement of the space shuttle in 2011, the USA has been relying on Russian Soyuz rockets, launched from Kazakhstan, to get astronauts to the ISS.
The successful flight marked the scheduled end of the 30-year space shuttle program.
The first SpaceX Crew Dragon flight to the space station will include NASA astronaut and Navy test pilot Victor Glover, who has logged more than 3,000 hours flying 40 different aircraft and NASA astronaut Mike Hopkins, who has logged 166 days in space so far.
The space station's operational lifespan has already been extended to reach 2025, though the White House recently began discussing the idea of turning the aging station over to commercial investors who might be interested in continuing the ISS's run of maintaining a continuously manned presence in space for almost two decades.
Since NASA retired its space shuttles, the agency has had to buy seats on the Russian Soyuz spacecraft to get its crews to the International Space Station. He made the introductions in a crowded auditorium at Johnson Space Center in Houston.
The Government Accountability Office said in a recent report that Boeing's spacecraft could "tumble" in some abort scenarios, which "could pose a threat to the crew's safety".
Josh Cassada, 45, will fly with Sunita ("Suni") Williams, 52, aboard Nasa's first contracted Starliner mission.
The timetable for the launches was supposed to have been for test flights to take place before the end of 2018, but both companies have faced technical issues that may delay that into 2019.
Since 2010, that program has inched along, moving ever-closer to crewed and uncrewed test flights.
NASA awaits the completion of both companies' test flights before certifying either safe for use to return astronauts on low-Earth-orbit missions. SpaceX's first operational flight to the space station will also be Glover's first flight to space. This will be her first trip to space.
Boeing's CST-100 Starliner will be launched from Space Launch Complex 41 at Cape Canaveral Air Force Station in Florida. Since her selection as an astronaut in 1998, she has spent 322 days aboard the International Space Station for Expeditions 14/15 and Expeditions 32/33, commanded the space station and performed seven spacewalks.