United Airlines is preparing to buy as many as 200 new single-aisle jets, and Boeing is the favorite to win the deal.

Aerospace sources are reporting that Airbus, Boeing’s rival in the contest for the big United order, has walked away from the negotiations, leaving Boeing and its 737 the sole contender for the business.

The United buy would be a big boost for Boeing which suffered a setback last summer when longtime Boeing-only customer American Airlines split a big order between Boeing and Airbus.

Since then, Boeing has won a large order from Delta and Southwest and several large overseas buyers for its 737.

United in recent years had turned its back on Boeing for single-aisle planes, buying Airbus’ A320s instead. With the merger of United and Continental into a single airline, Continental executives have emerged in several key positions in the merged airline. Continental had bought Boeing 737s for its single-aisle fleet.

At list prices of $95 million for a new 737 Max 8, the order for 200 would be worth some $19 billion, but the prices airlines pay in large orders is often significantly less than the list.

And industry reports differ on the number of planes United will order. Some say 100. Others are report 180 to 200 aircraft.

Aerospace observers are speculating the order announcement will come sometime later this week. Boeing builds all of its 737s at its Renton plant.