BOSTON — NASA announced July 9 it has selected four veteran astronauts to train for test flights on commercial crew vehicles under development by Boeing and SpaceX.

NASA Administrator Charles Bolden said in a statement that astronauts Robert Behnken, Eric Boe, Douglas Hurley and Sunita Williams will start training for the initial test flights of Boeing’s CST-100 and SpaceX’s Dragon crew spacecraft, currently scheduled for 2017.

All four astronauts are veterans of the shuttle program, serving as mission specialists or pilots. Williams flew two long-duration missions on the International Space Station, including one where she traveled to and from the station on Soyuz vehicles. Behnken was, until recently, chief of NASA’s astronaut office.

In an interview here during the ISS Research and Development Conference, Williams said she expected training to start in the next few months, but didn’t know yet the details of the training program. “We’re going to start working really closely with the commercial crew program,” she said. “The details of the whole thing have not been outlined yet.”

Williams said that she and the others will train on both commercial crew vehicles, and no crew assignments for specific test flights have been made yet. Boeing’s commercial crew contract includes a test flight with one NASA astronaut and one Boeing test pilot, while SpaceX plans to use two NASA astronauts for its test flight.

While the companies can tailor their flight test plan, NASA has a series of requirements for the crewed test flight included as milestones in both companies’ contracts. Those requirements include carrying at least one NASA astronaut, docking with the space station, remaining there long enough to check out station interfaces, and then returning to the desired landing site.

Both companies’ schedules call for carrying out those test flights in 2017 as some of the last major milestones before being certified by NASA to transport crews to and from the ISS. However, NASA has warned in recent months that the schedule could be in jeopardy if the agency does not get the $1.24 billion it requested for the program in fiscal year 2016. Current versions of House and Senate appropriations bills provide $1 billion and $900 million, respectively, for the program.

Bolden made a point of emphasizing the need for full funding of the program in his statement. “Every dollar we invest in commercial crew is a dollar we invest in ourselves, rather than in the Russian economy,” he said, referring to NASA’s current need to purchase Soyuz seats from the Russian space agency Roscosmos.