The third-largest U.S. carrier's order brings Boeing's total 787 orders for 2018 to 105 aircraft, already surpassing the 94 orders it received in all of 2017, a source familiar with the deal said.
The expanded routes are part of United's plans to increase its capacity for this year by around 4.5 percent to 5 percent, a quicker growth rate than other airlines in the industry.
Reuters reported in May that United was in talks with plane makers Airbus and Boeing over the purchase of wide-body, long-haul passenger jets to replace a fleet of 50 Boeing 767 aircraft.
United said on Monday it was increasing flights between New York and Los Angeles and San Francisco to 27 daily flights. Its new 787-10 jets, the newest and longest variant in Boeing's Dreamliner family, will enter service early next year.
The carrier said it would be the first North American airline to operate the 787-10 Dreamliner on select flights between those cities.
(Tracy Rucinski and Eric M. Johnson - Reuters)