United Airlines was founded in 1926 as Varney Air Lines, named after its founder, Walter Varney. It was originally a small air mail carrier that operated routes in the western United States. In 1929, Varney Air Lines was acquired by William Boeing, the founder of the Boeing Company, and was renamed Boeing Air Transport.

In 1931, Boeing Air Transport merged with Pratt & Whitney to form United Aircraft and Transport Corporation (UATC). UATC was a holding company that owned several aerospace and transportation companies, including Boeing Airplane Company, Pratt & Whitney, and United Air Lines. The merger allowed United to expand its operations to include passenger services, and it quickly became one of the largest airlines in the country.

In 1934, UATC was forced to divest itself of its airline subsidiaries due to regulatory pressure from the federal government. The airline operations were separated from the rest of UATC and reorganized as United Air Lines, Inc.

Throughout the 1930s and 1940s, United continued to expand its operations, adding new routes and acquiring other airlines. In 1961, it became the first airline to offer coast-to-coast service in the United States, with flights from Los Angeles to New York. In the 1970s, United was one of the first airlines to introduce widebody jets, such as the Boeing 747, which allowed it to offer more comfortable and spacious seating for long-haul flights.

In the 1980s, United was faced with increasing competition from low-cost carriers and underwent a series of changes in an effort to remain competitive. In 1986, it introduced a new fare structure that included lower prices for advance-purchase tickets, and in 1992, it launched a low-cost airline subsidiary called Ted (short for "Trans World Express"). Ted operated flights within the United States and to international destinations, using a fleet of Airbus A320 aircraft.

In 2010, United merged with Continental Airlines, creating the largest airline in the world at the time. The merged airline retained the United Airlines name and continued to operate under the United brand.

One of the most significant events in the history of United Airlines was the September 11, 2001 terrorist attacks. Two of the planes that were hijacked and flown into the World Trade Center towers were United Airlines flights: Flight 175 from Boston to Los Angeles, and Flight 93 from Newark to San Francisco. The attacks resulted in the deaths of all passengers and crew members on board the flights, as well as thousands of people in the towers.

The September 11 attacks had a profound impact on the airline industry and United Airlines was no exception. The company faced financial difficulties in the aftermath of the attacks and was forced to file for bankruptcy protection in 2002. It emerged from bankruptcy in 2006 and has since undergone several restructuring efforts in order to remain competitive in a rapidly changing industry.

Today, United Airlines is a major global airline, with a fleet of more than 700 aircraft serving destinations in over 50 countries around the world. It is a founding member of the Star Alliance, a global airline alliance that offers its customers a wide range of travel options and destinations. Despite the challenges it has faced over the years, United remains one of the most respected and well-known airlines in the world, known for its commitment to safety, reliability, and customer service.