OTTAWA — Bombardier turned to Airbus on Monday to save an airliner that has been surrounded by uncertainty even before it was hit with a potentially crippling trade action by Boeing in the United States.

In an unexpected announcement, Airbus and Bombardier, which is based in Montreal, formed a partnership on Monday to make and sell the CSeries airliner. Airbus will hold 50.1 percent of the venture but will not make any payment, future investment in the project or assume any debt related to the airliner, which has cost more than $5 billion to develop and put into production.

The arrangement gives Bombardier a way to make an end-run around two preliminary trade rulings that, if finalized, would impose tariffs that would more than quadruple the cost of the airliner in the United States, its key market. The two companies said they would move swiftly to open a CSeries assembly line in Mobile, Ala., where Airbus already makes its A320 airliners, that would most likely allow planes made there to avoid the enormous import duties.

Tom Enders, the chief executive of Airbus, and Alain Bellemare, his counterpart at Bombardier, told reporters that the agreement was not related to the trade action brought by Boeing. But they also said that many airlines had shied away from the CSeries because of concerns about the future of the project, which has been troubled by cost overruns and delays, and the future of Bombardier itself.