A top Boeing executive said today that the company plans to start sending crews into orbit aboard its CST-100 Starliner space taxi in 2018, which represents a slight delay in NASA’s previous development schedule.

“We’re working toward our first unmanned flight in 2017, followed by a manned astronaut flight in 2018,” Leanne Caret, who is Boeing’s executive vice president as well as president and chief executive officer of Boeing’s defense, space and security division, said at a briefing for investors.

Previously, Boeing said both test flights, uncrewed and crewed, were scheduled for 2017. Just this week, Aviation Week reported that Boeing was sticking to the 2017 schedule, even though it’s been working through challenges related to the mass of the spacecraft and aeroacoustic issues related to integration with its United Launch Alliance Atlas 5 launch vehicle.

In a follow-up to Caret’s comments, Boeing spokeswoman Rebecca Regan told GeekWire that those factors contributed to the schedule slip. In addition, NASA software updates have added more work for developers.

Boeing and SpaceX are building spacecraft to transport astronauts to and from the space station for NASA, under the terms of $6.8 billion in contracts that were awarded in 2014. Boeing was allotted $4.2 billion to work on the Starliner, while SpaceX is getting $2.6 billion to upgrade its Dragon cargo capsule to carry astronauts.

Each space taxi is designed to carry up to seven astronauts plus cargo, at a cost that’s less than the $81 million per seat that the Russians are charging for trips on Soyuz capsules. A smaller crew is expected to fly on the initial missions.

SpaceX conducted a pad abort test of its Crew Dragon (a.k.a. Dragon V2) a year ago. An uncrewed test flight is planned during the first half of 2017, followed by an in-flight abort test and a crewed Dragon flight to the space station later in the year.

This month, Boeing’s team bolted together a structural test version of the Starliner, which will be used to make sure the spacecraft can stand up to the rigors of spaceflight. Astronauts are already training on Starliner flight simulators, and components for the first flight vehicle will soon be put together inside Boeing’s processing facility at NASA’s Kennedy Space Center in Florida.

For Boeing to shift its crewed test flight from 2017 to 2018 isn’t as much of a slip as it might sound: The company’s earlier schedule had called for the visit to the space station to take place in mid-December.

However, if both companies stick to their stated schedules, SpaceX would become the first U.S. commercial venture to send astronauts to the space station – and as a result would take possession of a highly prized trophy: a U.S. flag that was left aboard the station by the last space shuttle crew in 2011.

Other news nuggets from Boeing Investor Day: