The purchase from France-based Airbus comes as Delta appears to be growing more distant from Boeing amid an intense trade dispute.
Boeing had complained to the Trump administration that Canadian airplane manufacturer Bombardier sold narrow-body C-Series jets to Delta below market prices and received unfair government subsidies. The U.S. Commerce Department sided with Boeing in the dispute over Bombardier's planes and recommended a duty of 300 percent on the some 100-seat C Series aircraft.
Delta has repeatedly said it would not pay the tariffs on the the plane and said Boeing's offer was for used planes. In a surprise move in October, Airbus swooped in and said it would take a majority stake in Bombardier's C Series program.
Word of Delta's Airbus order was reported earlier by CNN.
Delta's planes are 17 years old on average and its decision on new single-aisle jets was highly anticipated.
In the midst of the dispute over the Bombardier C-Series planes in October, Delta CEO Ed Bastian told reporters that it hasn't ruled out purchasing aircraft from Boeing even though the company launched a complaint over the deal.
"Yes, we will be taking more Boeing aircraft," he said.
Samuel Engel, head of the aviation practice at consulting firm ICF said it may not be game over for Boeing.
"It's a loss to Boeing if you think it's a trend but if it's just one data point, there's still room for more orders," he said. "The question is: 'What's the next order?'"
Big order book
Boeing shares rose 0.7 percent in late afternoon trade Wednesday, little changed after the news of the Delta order broke. The company announced an $18 billion buyback and raised its dividend by 20 percent earlier this week, adding fuel to its 2017 rally. Shares are up more than 87 percent so far this year.
The aircraft manufacturer has a robust order book with 661 net orders in 2018, most of them for single-aisle 737s.
Boeing has a "very healthy balance sheet and production rate outlook," said Richard Aboulafia, vice president at aerospace and defense analysis firm Teal Group.
Correction: An earlier version of this article misstated the location of Boeing's headquarters.