Updated 12:25 p.m. May 13.

WASHINGTON — Boeing has delayed the first crewed flight of its CST-100 Starliner commercial crew vehicle until early 2018, later than NASA’s original deadline, because of a series of technical issues and new requirements that the company argues are typical for an aerospace development program.

The current development schedule, Boeing spokeswoman Rebecca Regan said May 12, calls for a pad abort test of the spacecraft’s launch abort system in October 2017. That will be followed by an uncrewed orbital test flight of the spacecraft in December 2017. A crewed flight, carrying one NASA astronaut and one Boeing test pilot to the International Space Station, is now scheduled for February 2018.

Those milestones are several months later than what Boeing and NASA previously planned. A NASA chart listing milestones for its Commercial Crew Transportation Capability (CCtCap) contracts, dated November 2015 and presented at a March 2 meeting of the NASA Advisory Council’s human exploration and operations committee, had the uncrewed CST-100 test flight scheduled for June 2017 and the crewed test flight in October 2017.

Regan said that several factors contributed to the new schedule, which Boeing recently presented to NASA. One issue was reducing the spacecraft’s mass. “It’s typical of any development program,” she said of that work, adding that the mass was now “in the box.”

A second issue is aerodynamics issues during launch and ascent of the CST-100 on an Atlas 5. Regan said Boeing has developed a “really viable option” for dealing with pressure on the spacecraft and launch vehicle, and plans to test it in a wind tunnel soon.

A final factor, she said, was that NASA gave Boeing additional software requirements for the spacecraft, which requires about three months of work to complete. “A combination of those things contributed to the revised schedule,” she said.

The revised schedule, including the slip of the first crewed mission to 2018, came to light in comments by Leanne Caret, Boeing executive vice president and head of the company’s defense, space and security division, at a company investor conference May 11 in Seattle. “We’re working towards our first unmanned flight in 2017 followed by a manned astronaut flight in 2018,” she said. Previously, Boeing had indicated it planned to carry out that initial crewed flight before the end of 2017, although she made no explicit mention of a delay.

Caret added that even with the revised schedule, Boeing still hoped to be the first company to fly its commercial crew vehicle. “It is our vision that the CST-100 will be the first of the new American capsules to take astronauts to space,” she said.

However, even before the latest delay, Boeing’s schedule was behind that of the other company with a CCtCap contract, SpaceX. The same milestone chart presented at the March NASA Advisory Council committee meeting listed an uncrewed flight of SpaceX’s Crew Dragon spacecraft in December 2016, followed by a crewed flight test in April 2017.

NASA spokeswoman Stephanie Martin said May 12 that NASA had not yet updated the schedule of CCtCap milestones for the two companies since the one presented in March. Boeing, she said, has completed 13 milestones to date, and SpaceX has completed 8. According to SpaceX, the company now plans to perform the uncrewed test flight in the second quarter of 2017 and the crewed test flight in the third quarter of 2017.

Regan said it was an “aspiration” for Boeing to be the first U.S. company to fly a crewed spacecraft to the ISS, in part because the company is the prime contractor for the station. “There’s a lot of pride at Boeing to do that,” she said, but added the company prioritized safety over schedule.

Although the CST-100 flight test schedule is delayed, Boeing is emphasizing the progress it is making on the overall program. The company announced May 11 it had mated the two parts of a structural test article of the spacecraft. That test article will soon be shipped to a Boeing facility in California for environmental and other tests.

The revised schedule means Boeing won’t meet NASA’s goal of being certified by the end of 2017. That certification, which allows the company to perform crew transportation missions to and from the station, requires a final review after the crewed test flight to the station. In the scheduled NASA presented in March, that certification review for Boeing was scheduled for late November 2016.

That delay could raise new questions from Congress about the commercial crew program. In contrast to previous years, where it was a hot topic in congressional hearings, the program rarely came up during House and Senate hearings about NASA’s fiscal year 2017 budget request. A spending bill approved by the Senate Appropriations Committee April 21 provided the program with $1.18 billion, the amount requested by NASA.

The report accompanying the bill, though, indicated some skepticism about schedules even before this latest delay. “Both companies now anticipate completing contracted milestones on a delayed schedule, closer to the anticipated launch dates, compressing already aggressive and optimistic schedules,” the report stated. “This leaves little margin for either provider to maintain schedule as unanticipated challenges emerge during vehicle production and testing.”