Boeing CEO David Calhoun said the company needs to acknowledge its mistake as the aircraft maker reels from a door-plug failure that has resulted in roughly 170 of its planes being grounded and spooked its customers.

In his first remarks since the accident, Calhoun indicated a misstep by the aircraft maker played a role. “We are gonna approach this—No. 1—acknowledging our mistake,” Calhoun said Tuesday in an address to employees just days after the incident on an Alaska Airlines flight.

“We’re gonna approach it with 100% and complete transparency every step of the way,” he said. “We are going to work with the [National Transportation Safety Board] who is investigating the accident itself to find out what the cause is.”

Calhoun didn’t specify what mistake he was referring to, and other executives who spoke cautioned against speculation. Boeing declined to elaborate. The Journal reviewed audio of their remarks at an all-hands meeting at the 737 factory in Renton, Wash.

Regulators have grounded about 170 Boeing 737 MAX 9 planes since Saturday after a door plug detached from a MAX 9 jet at 16,000 feet, leaving the plane flying with a gaping hole. United Airlines and Alaska Airlines said they have found other MAX 9 planes with loose parts as they check the grounded fleet.

Boeing executives said airlines are shaken by the incident. “Moments like this shake them to the bone, just like it shook me,” Calhoun said in the meeting. “They have confidence in all of us—they do—and they will again.”

Calhoun said Boeing engineers are poring over information in search of clues of what went wrong.

Officials at the NTSB are still investigating the cause of the incident, and have said it is too soon to determine whether a manufacturing flaw might have led to the Alaska plane’s blowout. On Monday, they said that four bolts designed to keep the door plug from moving off the stops were missing, adding that they would work to determine whether the bolts had ever even been installed.

Calhoun, a former Boeing chairman and GE executive, took over as Boeing’s CEO at the start of 2020 when the company was enmeshed in a prior crisis over a smaller version of the plane. The MAX 8 was grounded for about two years after a pair of crashes in 2018 and 2019 that killed 346 people. Those accidents were blamed on a faulty flight control system that sent the planes into fatal nosedives and not related to door plugs.

After the earlier MAX crashes, Boeing faced criticism from U.S. lawmakers and regulators over disclosures related to the aircraft’s certification. The Justice Department tried a former Boeing pilot on charges he misled the FAA. A jury acquitted him.

In the wake of Friday’s incident, Boeing executives have scrambled to reassure airlines about the safety of its planes and are working to develop an inspection process so that grounded planes can be cleared for service. The Federal Aviation Administration said Tuesday that Boeing was revising its instructions and the regulator still hasn’t signed off on the process so the planes will remain grounded.

Calhoun said lessons derived from Friday’s incident will help improve aviation safety. “MAX has been on a journey—no doubt about it,” Calhoun said, alluding to problems other aircraft types have experienced. “We work our way through them—we do it diligently.”

Boeing director David Joyce, who chairs the board’s safety committee, said the panel met on Saturday and would be following the investigation closely as it proceeds, getting regular reports from Calhoun and other top company leaders.

The loose hardware found by United and Alaska after the accident are the latest in a series of quality problems that have surfaced in various types of Boeing commercial and military aircraft in recent years. Boeing has sought to improve how it approaches safety, engineering and production after the earlier MAX crashes.

Boeing commercial airplanes chief Stan Deal said Tuesday the plane maker didn’t know whether the recently discovered defects had any links with Friday’s accident. “We build the airplane and we have to own it,” Deal said. “We’ve come a long way in this journey, but we’re not at an end.”

—Sharon Terlep contributed to this article.

Write to Andrew Tangel at