Boeing had been working to strengthen engine covers on the 777 for about two years before last weekend's scare on a United Airlines flight, according to a report Thursday.
Boeing was working with the Federal Aviation Administration to strengthen protective engine covers following similar problems on earlier flights preceding Saturday's emergency landing, the Wall Street Journal reported.
The report comes amid a National Transportation Safety Board investigation of Saturday’s United flight, which returned to Denver soon after disembarking after the engine caught fire and began breaking apart.
No one was hurt in the incident, but the episode raised questions about maintenance on the jets.
On Tuesday night, the FAA ordered inspections of all Pratt & Whitney engines similar to the one that broke apart.
Investigators have attributed the Denver incident to a fan blade that broke off soon after takeoff due to metal fatigue and apparently breached the engine cover, known as a cowling.
Both Boeing and the FAA avoided discussing specifics on the efforts to modify the 777. Such changes typically require signficant evaluation and testing.
Boeing is "in constant communication with our customers and the FAA, and engaged in ongoing efforts to introduce safety and performance improvements across the fleet," a Boeing spokesman said.
"We will continue to follow the guidance of the FAA on this issue and all matters related to safety and compliance, and we continue to provide updates to our customers."
The FAA said it focused on fan blade inspections in its most recent order on the Pratt & Whitney engines and on an earlier directive after a 2018 incident on a 777.
"Redesigning airframe and engine components is a complex process. One of the top priorities to date has been reducing the risk of a fan-blade failure that could lead to cowling damage," an FAA spokesman said.
"The FAA engages with manufacturers to continuously enhance safety. Any proposed design change to a critical piece of structure must be carefully evaluated and tested to ensure it provides an equivalent or improved level of safety and does not introduce unintended risks."