Dennis Muilenburg stepped down as Boeing Co. CEO, effective immediately, as the airline giant struggles to recover from the fallout of two fatal crashes of its now-grounded 737 MAX jets.

"The board of directors decided that a change in leadership was necessary to restore confidence in the company moving forward as it works to repair relationships with regulators, customers, and all other stakeholders," the company said in a statement Dec. 23. Muilenburg also exited the company's board.

The board appointed Chairman David Calhoun as CEO and president, effective Jan. 13, 2020. CFO Greg Smith will be interim CEO until then.

Additionally, board member Lawrence Kellner will be nonexecutive chairman, effective immediately, Boeing said.

The tumult started with crashes of 737 MAX flights operated by Lion Air and Ethiopian Airlines in October 2018 and March 2019, respectively. A fault in the 737 MAX's Maneuvering Characteristics Augmentation System, or MCAS, was found to have contributed to the crashes. In a recorded statement released in April, Muilenburg said "erroneous activation of the MCAS function can add to what is already a high-workload environment." The Federal Aviation Administration ordered the planes grounded in March.

Boeing shares were up 2.7% at the start of the trading day Dec. 23, at $336.75.