Commercial Crew Program Manager Kathy Lueders gave an update to the NASA Advisory Council on Monday.
SpaceX is making progress on its Dragon 2 crew vehicle.
Two Dragon flight tests — one with a crew, the other without — are set to fly to the International Space Station.
The system would then undergo certification to carry NASA astronauts on a commercial basis.
Boeing is working on the CST-100 Starliner spacecraft.
Starliner will fly aboard an United Launch Alliance (ULA) Atlas V.
Like Dragon, the Starliner will carry four astronauts on a standard mission but could carry as many as seven.
Both Boeing and SpaceX are set to conduct their flights tests this year, although there is a strong possibility both could slip into 2019.