United Airlines confirmed Thursday that it will seek bids from Boeing Company and rival Airbus for a major jet order that could be placed as soon as this autumn. The third-biggest U.S. airline said it will examine proposals to replace its wide-body fleet over several years, as well as its Boeing 757s. Unlike other major airlines, United has not been ordering new airplanes over the past few years. Instead, it has been shrinking to attempt to match its flight operations with reduced demand, and it is working to eliminate its Boeing 737 fleet. United chief executive Glenn Tilton told workers in a message Thursday that the company has had ongoing discussion with Boeing and Airbus over several years but decided not to order new airplanes “until we believe we can generate a return on our investment.” “The analysis we have conducted for more than a year suggests that time may be now,” Tilton said. Airbus and Boeing have seen declining orders for passenger and cargo jets due to the global economic slump, which has reduced demand for air travel. Tight credit markets also have made it more difficult for airlines to secure financing for new airplanes.