The planemaker will also deploy a team to supplier Spirit AeroSystems - which makes and installs the plug door involved in the incident - to check and approve Spirit's work on the plugs before fuselages are sent to Boeing's production facilities in Washington state, Stan Deal, president of Boeing Commercial Airplanes, said in a letter to Boeing employees.

The new actions from Boeing come after the Federal Aviation Administration on Friday extended the grounding of 171 MAX 9 planes indefinitely for new safety checks. Only after 40 planes are inspected will the agency review the results and determine if safety is adequate to allow the MAX 9s to resume flying, the FAA said.

In addition to the door plug inspections, Boeing teams will conduct checks at 50 other points in Spirit's production process, Deal said. Meanwhile, both Boeing and Spirit will open their 737 production facilities to airline customers for carriers to provide their own inspections.

Boeing will also hold sessions for employees on quality management, and bring in an outside party to conduct an independent assessment of its production process, Deal said.

Deal said the actions laid out in the letter are separate from the FAA's ongoing investigation and plans to increase oversight of MAX production.

However, before new MAX 9s are delivered, Boeing "will conduct the same thorough inspections of the mid-exit door plugs as mandated by the FAA," Deal wrote.

The regulator announced last week it will also audit the Boeing 737 MAX 9 production line and suppliers and consider having an independent entity take over certain aircraft certification responsibilities the FAA previously assigned to the planemaker.

Boeing has boosted its number of a quality inspectors by 20% since 2019 and plans to make additional investments to its quality units, Deal wrote.

"Everything we do must conform to the requirements in our QMS (quality management system)," Deal wrote. "Anything less is unacceptable. It is through this standard that we must operate to provide our customers and their passengers complete confidence in Boeing airplanes."

United Airlines and Alaska Airlines have canceled all MAX 9 flights through Tuesday.