United Airlines, Inc. is a major American airline headquartered at the Willis Tower in Chicago, Illinois.[10][11][12] United operates an extensive domestic and international route network across the United States and all six inhabited continents[13] primarily out of its eight hubs, with Chicago-O'Hare having the largest number of daily flights[14] and Denver carrying the most passengers in 2023.[15] Regional service is operated by independent carriers under the brand name United Express.