Boeing will be the first commercial company to carry a NASA astronaut to space in July 2017 under a contract with the US space agency, followed by its competitor SpaceX.

NASA is funnelling billions of dollars to both companies so that they can replace American access to the orbiting International Space Station after the US space shuttle program was retired in 2011.

The announcement of US$4.2 billion for Boeing and US$2.6 billion for SpaceX was made in September 2014.

However, a legal challenge by Sierra Nevada - which was developing a space-shuttle-like vehicle called Dream Chaser and was closed out of the competition - meant that officials could not reveal many details until now.

The US Government Accountability Office denied the protest by Sierra Nevada earlier this month, allowing NASA and its partners to speak publicly about the contracts and future plans for test flights.

Commercial crew program manager Kathy Lueders said at a press conference in Houston, Texas that Boeing would be the first to make two contracted missions to carry NASA astronauts, since it has completed two milestones so far, and SpaceX just one.

"Our goal is to have two robust providers," Lueders added.

A NASA astronaut and a Boeing test pilot will make the first crewed test flight on the spacecraft called Crew Space Transportation-100, or CST-100 for short, in July 2017, said John Elbon, Boeing's vice president and general manager of space exploration.

"The first services mission then will begin in December of '17," he told reporters, referring to the first official trip to the International Space Station with crew on board.

SpaceX's upgraded Dragon crew vehicle is on track for an unmanned test flight in 2016, followed by a test flight including crew on board in early 2017, said vice-president Gwynne Shotwell, who did not give specific dates.

Read more:

NASA funds Boeing, SpaceX to carry US astronauts