Boeing's 737 MAX plane is "designed by clowns who in turn are supervised by monkeys", a company employee wrote in one of a trove of newly published internal messages.
The communications were released on Thursday by Boeing, whose 737 MAX plane was involved in two crashes that killed a total of 346 people in late 2018 and early 2019.
They include employees mocking US aviation authorities and bragging that they could get the plane certified with minimal training for pilots.
The documents could further worsen Boeing's relations with regulators as it works to secure approval to allow the grounded jetliner to resume flights.
"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 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.
Boeing said in a statement: "Some of these communications relate to the development and qualification of Boeing's MAX simulators in 2017 and 2018."
The messages were sent to congressional investigators in the interest of transparency, it added.
The mocking tone of the messages is yet another embarrassment for Boeing amid the 737 MAX crisis and worsens its already rocky relations with the FAA. The plane has been grounded worldwide since March 13.
"These newly released emails are incredibly damning," said Peter DeFazio, the chairman of the House Transportation Committee, which is investigating the plane.
"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."
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