Boeing is competing with Space Exploration Technologies Corp, or SpaceX, and privately held Sierra Nevada Corp, to develop and build U.S. commercial space taxis to transport astronauts, rather than relying on Russia to ferry them to the station.
The multibillion-dollar program has taken on new urgency in recent months, given escalating tensions with Russia over its annexation of the Crimea region of Ukraine.
Nasa spokeswoman Stephanie Martin said the U.S. space agency planned to choose one or more of the competitors to continue working on the program in late August or early September.
Martin confirmed that Boeing had completed a critical design review of its offering in the Commercial Crew Integrated Capability (CCiCap) program. She said Nasa was reviewing the data to determine if Boeing met its required "success criteria" for the review.
SpaceX and Sierra Nevada have sought and won extensions to finish their design reviews by May 2015. Blue Origin, a privately funded company set up by Amazon.com founder Jeff Bezos is also vying for the work.
John Mulholland, vice president and program manager for Boeing's commercial space exploration programs, said the company faced some difficult challenges as it developed its design, but got "excellent" feedback from Nasa during the review last month.
"From a technical standpoint, the review went very well," he said. "To the best of my knowledge we're the only CCiCAP competitor that actually was able to complete all of the milestones in the period of performance," he said.
Mulholland said, measured in mass, the Boeing design for the cargo module was 96-percent complete at the time of the review, while its design for the crew module was 85-percent complete, two metrics that underscored the maturity of the design.
He said the critical design review marked a major step for the Boeing program. "You've got to be able to stand up at that review and show the analysis and tests that demonstrate that you're going to be able to meet those requirements," he said.
Boeing remains confident it could complete work on the new spacecraft in time to begin flight tests in 2017, Mulholland said.
He said Boeing's design would be launched into space using the Atlas 5 rocket built by the United Launch Alliance, a venture of Boeing and Lockheed Martin Corp .
That rocket uses a Russian-built RD-180 engine, which has also triggered some concerns given tensions with Russia.
He said the module was designed from the beginning to be compatible with other launch vehicles, if necessary, although that would still entail some modification of the interface between the spacecraft and the launcher.
© Thomson Reuters 2014