Boeing said it has flown 737 MAX with updated software for the Maneuvering Characteristics Augmentation System, thought to be a factor in both crashes, for more than 360 hours on 207 flights.
"Boeing did not treat the 737 Max 8 situation like the emergency it was", said Daniel Carey, president of the American Airlines pilots' union, which has filed public records related to the matter.
"We will not allow the 737 MAX to fly in the USA until it is absolutely safe to so", he said.
When asked why the FAA approved a system that could cause the aircraft to dive based on one faulty angle-of-attack (AOA) reading, he replied that pilots could counter MCAS by using a checklist "they should have in memory".
He said the two accidents have only intensified Boeing's commitment to its values, including safety, quality and integrity, because "we know lives depend on what we do".
Boeing has also developed training and educational materials in a bid to support their return to service.
Boeing engineers have been working on the software update for more than six months - far longer than they expected - having started shortly after the October crash of a MAX operated by Indonesia's Lion Air.
This includes a series of regional customer conferences being conducted around the world, the statement said.
Last week, Seifman met with Boeing management with a focus, obviously, on the 737 Max.
A file image of the Boeing 737 MAX tail and winglets.
Meanwhile, multiple investigations, including the initial crash investigation, are ongoing.
"I would call it the beta version", Lawrence said.
The FAA will need to send the software to the FAA and FAA equivalents in other countries. "The reason why they submitted it to us is so we can stick it in the simulator so we could test it, so we can also look at their system safety analysis and see whether it will appropriately address it".
Boeing earlier triumphantly announced that it has implemented new failsafe features for the MCAS as well as new training manuals for pilots.
Aviation regulators in other countries will complete their own reviews of the software separate from the global Joint Authorities Technical Review, or JATR, the FAA has organized.