"Space has transformed the American way of life", NASA Administrator Jim Bridenstein said at the event.
The missions will mark the first crewed launches from USA soil since the end of the space shuttle programme in 2011. "Today's announcement advances our great American vision and strengthens the nation's leadership in space". NASA worked closely with the companies to engineer both spacecraft.
Leading up to the ceremony at the Johnson Space Center here, Bridenstine said it was a historic moment for the agency: "We are going to launch American astronauts from American soil".
In 2014, SpaceX and Boeing received contracts for $2.6 billion and $4.2 billion, respectively, to develop space capsules that can ferry astronauts to and from the space station.
If those missions go well, astronauts Mike Hopkins and Victor Glover will fly the first operational Crew Dragon flight to the International Space Station, while astronauts Sunita Williams and Josh Cassada will fly the first operational CST-100 Starliner mission.
Both Boeing and SpaceX have faced setbacks and delays - with the latest schedule slips being announced on Thursday. It'll be his first spaceflight, though he has a wide range of experience with combat missions and testing aircraft. He's spent almost 29 days in space.
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Veterans of two spaceflights each, Mr. Behnken and Mr. 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 Mr. Hurley as pilot.
"SpaceX is targeting November 2018 for Crew Dragon's first demonstration mission and April 2019 for Crew Dragon's second demonstration mission, which will carry two NASA astronauts to and from the International Space Station", said Benji Reed, director of Crew Mission Management at SpaceX, in the statement.
Since the shuttle was retired seven years ago, US astronauts have had to fly on Russian rockets from a remote launch site in Kazakhstan.
Victor Glover: Glover was selected as an astronaut in 2013 after a career in the Navy as a test pilot.
The SpaceX crewed test flight, now scheduled for April 2019, will be flown by NASA astronauts Robert Behnken and Douglas Hurley.
NASA has named the astronauts who will fly the first commercial capsules into orbit next year. John Mulholland, vice president and program manager for Boeing's Commercial Crew effort, explained that the earliest the company can "confidently" do a manned flight test will be in mid-2019. This will be her first trip to space. While there's talk of pushing NASA's last Soyuz return to January 2020, the space agency won't extend the Soyuz agreement forever.
"The 7,000 women and men of SpaceX understand what a sacred honor it was for us to be part of this program and for us to fly you", Shotwell told the astronaut crews.
For the past seven years, Nasa astronauts have hitched rides to the orbiting outpost on Russia's Soyuz spacecraft - at a cost of some US$80 million a seat.