Butch Wilmore and Suni Williams will blast off on the years-delayed flight on May 6 for a weeklong stay on the International Space Station (ISS).

"We're about to launch on Boeing Starliner and broaden the capability to and from Space Station that our agency has, and that's vitally important," Wilmore told reporters.

Since 2020, NASA astronauts have traveled to and from the ISS via SpaceX's Dragon craft.

The US space agency has long hoped to add a second transportation provider, but Boeing has struggled with a number of delays in developing the Starliner.

The space capsule finally managed to arrive at the ISS in May 2022 -- without a crew on board.

Now, it must carry out a crewed certification flight -- which was originally scheduled for 2022 -- before it can officially begin its transport missions.

Both astronauts have already visited the ISS, with Williams saying she has "all the confidence" in the ship and the mission control team.

"There is so much with... the capabilities of this spacecraft that other spacecraft don't have," Wilmore added.

The rocket that will propel the Starliner into space is an Atlas V by the United Launch Alliance (ULA), a joint venture between Boeing and Lockheed Martin.