Chicago ? United Airlines is exploring replacing its aging fleet of Boeing wide-body jets and has formally asked Boeing Co. and Airbus SAS to submit their best offers for its business.

United is the only major U.S. carrier that hasn’t ordered new jets since the airline industry entered a tailspin following the Sept. 11, 2001 terrorist attacks.

Although United has held ongoing talks with Boeing and Airbus in recent years, the carrier said it preferred to upgrade the first-class cabins of its existing jets rather than take on the debt and financial risk that accompanies a large aircraft order.

But in recent weeks, the nation’s No. 3 carrier decided to take advantage of a global aviation collapse that has left Boeing and Airbus scrambling to find takers for some of the large jets rolling out of their factories.