Boeing internal documents show that its employees mocked US aviation regulators and bragged they could get the 737 MAX certified with minimal training for pilots.
The communications, made public by US politicians, include exchanges among Boeing test pilots that speak of problems with flight simulators reproducing actual flight conditions, the company said.
The documents could further worsen Boeing's relations with regulators as it works to get the grounded jetliner back in operation.
"I still haven't been forgiven by God for the covering up I did last year," one employee wrote in a message from 2018 in reference to dealing with the Federal Aviation Administration (FAA).
"I know but this is what these regulators get when they try and get in the way. They impede progress," another wrote in August 2015.
"This airplane is designed by clowns, who in turn are supervised by monkeys," said yet another employee in 2017, apparently in reference to the FAA.
"Would you put your family on a MAX simulator trained aircraft? I wouldn't," a Boeing employee wrote to a colleague in another exchange.
"No," the colleague answered.
The emails and text messages were made public by US lawyers investigating the certification of the 737 MAX, which was involved in two crashes that killed a total of 346 people in late 2018 and early 2019.
"Some of these communications relate to the development and qualification of Boeing's MAX simulators in 2017 and 2018," Boeing said in a statement.
The company said it sent them to politicians in the interest of transparency.
The mocking tone of the messages is yet another embarrassment for Boeing amid the 737 MAX crisis and worsens its already poor relations with the FAA.
The plane has been grounded worldwide since 13 March.
Probes of the two crashes have focused in particular on the Maneuvering Characteristics Augmentation System, an automated flight control system.
Boeing is now working on changes to that system demanded by the FAA.
In late December, Boeing pushed out its embattled chief executive, Dennis Muilenburg, because of tensions with the FAA and replaced him with board chairman David Calhoun.
The FAA said while the "tone and content of some of the language contained in the documents is disappointing," the emails did not reveal any new safety risks.
"Our experts determined that nothing in the submission pointed to any safety risks that were not already identified as part of the ongoing review of proposed modifications to the aircraft," an FAA spokesman said.
"We continue to work with other international aviation safety regulators to review the proposed changes to the aircraft. Our first priority is safety, and we have set no timeframe for when the work will be completed."
Representative Peter DeFazio, the Oregon Democrat leading the House of Representatives investigation of the MAX, called the emails "incredibly damning."
"They paint a deeply disturbing picture of the lengths Boeing was apparently willing to go to in order to evade scrutiny from regulators, flight crews and the flying public."