Dennis Muilenburg, CEO of Boeing Inc. speaking at the Business Roundtable CEO Innovation Summit in Washington D.C. on Dec. 6th, 2018.

The Boeing 737 Max, grounded for the last six months after two fatal crashes, might not return to service in every country on the same timeline, depending on when global regulators deem the plane airworthy, CEO Dennis Muilenburg said Wednesday.

The manufacturer has developed a software fix for the jets, its best-selling plane, but hasn't yet submitted it to regulators for approval.

Muilenburg told an industry conference that Boeing still expects the planes to return to service early in the fourth quarter, but regulators across the globe may not clear it for flight at the same time.

"I think the phased ungrounding of the airplane amongst regulators around the world is a possibility," he said.

Boeing shares were up by about 3% in afternoon trading.