WASHINGTON (AP) -- Boeing attempted a major step Friday toward getting its 787 Dreamliners flying again, proposing a fix for the plane's troubled batteries that could allow the flights to resume as early as April, congressional officials said.

The next question is whether the Federal Aviation Administration will agree to let the planes fly even though the root cause of a battery fire in one plane and a smoking battery in another is still unknown.

A Boeing team led by CEO Ray Conner presented the plan to Federal Aviation Administration head Michael Huerta. The airliners, Boeing's newest and most technologically advanced, have not been allowed to fly since mid-January.

The plan -- a long-term solution, rather than a temporary fix -- calls for revamping the aircraft's two lithium ion batteries to ensure that any short-circuiting that could lead to a fire won't spread from one battery cell to the others, officials said. That would be achieved by placing more robust ceramic insulation around each of the battery's eight cells. The aim is to contain not only the short-circuiting, but any thermal runaway, a chemical reaction that leads to progressively hotter temperatures.

The additional spacers will enlarge the battery, requiring a bigger battery box to hold the eight cells. That new box would also be more robust, with greater insulation along its sides to prevent any fire from escaping and damaging the rest of the plane, officials said.

The plan will require testing and partially recertifying the safety of the plane's batteries, said the officials, who spoke on condition of anonymity because they weren't authorized to speak publicly.

The testing and recertification will take time, with engineers currently estimating completion sometime in April at the soonest, they said. Even after the batteries are recertified, it could take some more time to get the planes back into the air. Boeing will have to send teams to seven airlines in six countries to retrofit their planes.

It's up to Huerta to decide whether to approve the plan. But Boeing's plan is not a surprise, since the company has kept regulators closely informed, the officials said.

"The FAA is reviewing a Boeing proposal and will analyze it closely,'' the agency said in a statement Friday. "The safety of the flying public is our top priority and we won't allow the 787 to return to commercial service until we're confident that any proposed solution has addressed the battery failure risks.''

Boeing also acknowledged the meeting, but spokesman Marc Birtel would not discuss what was said. "We are encouraged by the progress being made toward resolving the issue,'' the company's written statement said.

Boeing, the FAA and the National Transportation Safety Board still have not identified the root cause of a Jan. 7 fire that erupted in an auxiliary power unit battery of a Japan Airlines 787 about a half-hour after the plane landed at Boston's Logan International Airport. The safety board is investigating that incident, but NTSB officials didn't attend Friday's meeting and declined to comment on the proposal.

Engineers and battery experts gathered by Boeing developed a list of possible causes for the fire and a plan to modify the batteries to address the spread of a fire created by any of those causes, officials said.

Nine days after the Boston fire, an All Nippon Airways 787 with a smoking battery made an emergency landing by in Japan. The FAA and aviation authorities overseas ordered the planes grounded soon afterward. There are a total of 50 of the planes in service worldwide, and Boeing had orders for 800 of the airliners at the time they were grounded.

On Thursday, United Airlines cut its six 787s from its flying plans at least until June and postponed its new Denver-to-Tokyo flights as airlines continued to tear up their schedules while the plane is out of service. United is the only U.S. carrier with 787s in its fleet.

The 787 is the world's first airliner made mostly from lightweight composite materials. It also relies on electronic systems rather than hydraulic or mechanical systems to a greater degree than any other airliner. And it is the first airliner to make extensive use of lithium ion batteries, which are lighter, recharge faster and can hold more energy than other types of batteries.

Boeing has billed the plane to its customers as 20 percent more fuel efficient than other mid-sized airliners. That's a big selling point, since fuel is the biggest expense for most airlines.

One question is how much weight Boeing's proposed fix would add. The heavier the plane is, the less fuel-efficient it is.

Having the plane flying as soon as April "would be fantastic news for Boeing,'' said Carter Leake, an aerospace analyst at BB&T Capital Markets.

If the battery fix ends up being as described, "I don't think it's that difficult to retrofit. I think it would be viewed very favorably'' by investors, Leake said. If FAA were to approve Boeing's proposed fix as early as next week, that would be a "home run'' for the company, he said.

However, the idea of recertifying part of the design is trickier. Getting certification from the FAA for a particular part or design is an involved process -- and one that's likely to make investors nervous.

"Recertification suggests time,'' Leake said. "Given what most know about aircraft certification processes, six months would be sort of quick.''

Leake added: "The FAA takes it slow. You're talking about statistical testing. You're proving through testing that this meets very stringent criteria. That usually involves time, and time is not on Boeing's side.''

Among the many unanswered questions is how the 787 battery problems will affect Boeing's effort to win FAA permission for the planes to make flights that venture further from the nearest airport, such as those that travel over wide expanses of ocean. The FAA has tighter requirements for such flights in twin-engine planes because it wants to make sure the plane can keep flying if it loses an engine or encounters other problems far away from a safe landing.

Until it was grounded, the 787 could fly up to three hours away from the nearest airport. That's far enough for flights between the U.S. and Europe and some flights over the Arctic, for instance. But Boeing wants permission for flights up to 5.5 hours from the nearest airport. Its 777 is already certified for such flights.

Boeing said last month before the grounding orders that it was close to submitting a plan for those longer flights.

The grounding has forced airlines that own the 787 to rework their schedules. LOT Polish Airlines has said the grounding of its two 787s is costing it $50,000 per day. Most affected has been ANA, which has 17 of the planes.

Boeing has had hundreds of people looking for the cause of the problem and working on possible solutions.

The mess comes just as Boeing is boosting 787 production from five planes per month to 10 per month by the end of this year. It has said the speedup will still happen, even though it can't deliver the planes -- or collect most of their $200 million-a-plane list price from airlines -- until they're flying again.

"Even with the FAA review/grounding, we believe it's more likely than not that Boeing continues to build at its planned rate until it's apparent that a fix for the battery issue will require an extended period of time (more than couple of months),'' UBS analyst David Strauss wrote in a note this week.