Long-discussed plans for a 323-seat version of the 787 are likely to be formally announced at next week's Paris Airshow, the sources said, confirming a Wall Street Journal report.
"We have no comment on the report but we are engaged in discussions with customers on a potential new member of the 787 family," a Boeing spokesman said.
The arrival of a new 787 version has been well anticipated, especially after Singapore Airlines Ltd recently made a provisional commitment to buy the new $300 million plane if Boeing decided to go ahead and build it.
Boeing's plans attracted attention on the eve of the maiden flight of the Airbus A350, which will compete for many of the same buyers. Both aircraft are made of lightweight carbon composites to help airlines save fuel. The A350 is expected to take to the skies in Toulouse, southwestern France on Friday.
British Airways parent IAG has an option to buy more Dreamliners and there has been speculation it will use some of these to buy the larger version, known as the 787-10.
U.S. lessor Air Lease Corp has been offering suggestions on the design and has publicly expressed interest in the 787-10. United Airlines has also backed it.
The sources, who asked not to be named, said other airlines could also be part of an early pool of buyers to give the plane a commercial boost. Later, Germany's Lufthansa is expected to look closely at the plane, they said.
The "stretched" 787-10 will have a longer fuselage and carry more passengers than the two Dreamliner models currently on the market, but offer less range. Boeing says that will suit airlines flying regional traffic across Asia or serving many points between continents.
Offering more seats without making airlines pay for range they do not need can make an aircraft more economic to run.
The airplane has been discussed for several years but took time to come to fruition as Boeing faced production delays on smaller models and a three-month grounding of its Dreamliner fleet earlier this year due to battery problems.
Airbus, owned by European aerospace and defense company EADS , and Boeing have clashed about the 787-10 even before the air show starts.
Airbus officials say stretching the 787 will force the aircraft to sacrifice too much range, repeating the fate of a previous-generation model, the 767-400ER, which failed to attract significant orders.
Boeing executives say the 787's base model starts with a much longer stride, having exceptional range due to its lightweight structure.
Growing competition for the next generation of wide-body jets is expected to dominate the show, which starts on Monday.