The precise size of the order could not immediately be determined but is expected to be significant and announced as soon as this month, the sources said.
The Wall Street Journal, which reported the plan earlier, said United was close to ordering dozens of 787 Dreamliners.
In October, Bloomberg News said United was nearing an order for more than 100 widebody jets and was evaluating offers from Boeing and for Airbus SE's (AIR.PA) A350.
In October, Boeing said it was continuing to produce 787s at a low rate, and "will gradually return to five airplanes per month over time." Boeing said with strong demand and more than 400 widebody airplanes in backlog "we anticipate higher production rates."
United and Boeing both declined to comment. United had 64 787 Dreamliners at the end of September and will have 68 by the end of 2022.
In August, Boeing delivered its first 787 since May 2021 after facing production problems. The Federal Aviation Administration cleared the way for deliveries to resume after the agency approved Boeing's inspection and retrofit plan needed to meet certification standards in July.
Both Boeing and Airbus have reported growing demand for widebody jets. United shares were down 1.3% in afternoon trade.
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