Boeing will resubmit documentation for its proposed 737 MAX software fix in a format requested by US and European regulators, but the aircraft builder said it's too early to determine if the move will further delay the jet's return to commercial service.

The review involves software for a second flight-control computer to be used on all future flights. The addition of a second computer is intended to make the system more reliable. In the past, one computer was used.

The MAX, Boeing's top-selling airplane, has been grounded since March after crashes in Indonesia and Ethiopia killed 346 aboard. Investigators believe the MAX's automated anti-stall device, called the Maneuvering Characteristics Augmentation System (MCAS), erroneously pointed the nose of the planes down to avoid a midair stall and into a fatal plunge.

"Boeing provided technical documentation to regulators as part of the software validation process," the company said in a statement to China Daily.

"The documentation was complete, and it was provided in a format consistent with past submissions. Regulators have requested that the information be conveyed in a different form, and the documentation is being revised accordingly.

"While this happens, we continue to work with the FAA (Federal Aviation Administration) and global regulators on certification of the software for safe return of the MAX to service."

The second computer will act as a backup and a check for the first. If the first computer malfunctions, the second can take over.

The new software will alert a flight crew if the computers report conflicting data about the plane's altitude, nose angle or speed.

It's unclear what additional pilot training will be required before approving the MAX's return to commercial service. Boeing hopes the FAA will approve the new software no later than December, but US airlines that use the plane don't expect it to return to service until January at the earliest and possibly February.

Meanwhile, Airbus, Boeing's chief rival, said it plans to boost production in China. The move is intended to improve the European plane builder's position in what is expected to become the world's largest aviation market and put additional pressure on Boeing as it struggles to get the MAX back in the air amid the US-China trade dispute.

Airbus plans to expand its A330 wide-body completion center in Tianjin so it can accommodate the larger A350 model. Airbus also plans to increase local production of the A320neo, the chief competitor to the Boeing 737 MAX.

The deal was outlined during a visit by French President Emmanuel Macron with President Xi Jinping in Beijing.

The plane builder's A320 plant in Tianjin is expected to produce six planes a month by the end of the year, earlier than the original target date of 2020.

In September, Boeing estimated that China will need more than 8,000 new aircraft valued at about $1.3 trillion in the next 20 years to meet demands of the quickly expanding Chinese middle class.

For years, Boeing and Airbus have increased production to meet growing demand for more fuel-efficient, narrow-body jets.

In the US, flight attendants for Southwest Airlines may join pilots in suing Boeing for lost wages.

Southwest has the largest US fleet of MAX jets and doesn't expect the plane to return to service until at least February.