Varney Air Lines (VAL), founded by Walter Varney, flew the first privately contracted air mail flight in the U.S. on April 6, 1926.

William Boeing founded Boeing Air Transport in 1927 to operate air mail routes under contract with the United States Post Office Department.

United Aircraft Corp. acquired mail and passenger service operator Pacific Air Transport on January 1, 1928, marking a significant step in the consolidation of carriers and equipment manufacturers that led to the emergence of United Airlines.

United Aircraft and Transport Corporation (UATC) acquired America's first scheduled passenger services carrier Stout Air Services on April 29, 1929, further strengthening its position in the aviation industry.

United Aircraft and Transport Corporation (UATC) acquired National Air Transport, a large Chicago-based mail-only carrier, on May 7, 1930, expanding its network and services.

On March 28, 1931, United Aircraft and Transport Corporation (UATC) formed the corporation United Air Lines, Inc. to manage its airline subsidiaries, marking a significant milestone in the company's history.

On the night of October 10, 1933, a United Boeing 247 exploded in mid-air and crashed near Chesterton, Indiana, killing seven people aboard. An investigation revealed that the explosion was caused by a nitroglycerin bomb placed in the baggage hold. The incident is believed to be the first proven case of air sabotage in commercial aviation history. No suspects or motives were ever found.

The Air Mail Act of 1934 banned the common ownership of manufacturers and airlines, leading to the restructuring of United Aircraft and Transport Corporation (UATC) into three separate companies, including United Air Lines.

United adopted a red, white, and blue shield logo in 1936, reflecting its brand identity and visual representation during that period.

On November 1, 1955, United Airlines Flight 629 was bombed while flying from Stapleton Airport in Denver to Portland, killing all 39 passengers and five crew members on board the Douglas DC-6B. The bomb was planted by Jack Graham, who put the device in his mother's luggage to collect on her life insurance policy. Graham was arrested, tried, and later executed a year after the explosion.

On June 30, 1956, United Flight 718 collided with a Trans World Airlines Lockheed L-1049 Super Constellation over the Grand Canyon, resulting in the world's deadliest commercial airline disaster at that time.

On April 21, 1958, United Flight 736 crashed in southern Nevada after colliding with a USAF F-100 Super Sabre fighter jet.

On December 16, 1960, United Flight 826 hit another TWA Super Constellation over New York City.

In June 1961, United merged with Capital Airlines, becoming the world's second largest airline behind Aeroflot of the Soviet Union.

In 1965, United Airlines introduced the slogan 'Fly the Friendly Skies' to replace its previous slogan 'The Main Line Airway.'

In 1968, United reorganized, creating UAL Corporation with United Airlines as a wholly owned subsidiary.

In August 1970, United took delivery of their first Boeing 747s, initially operating them on longer routes within the United States.

On December 26, 1973, then-president Richard Nixon flew aboard a United DC-10 flight from Washington Dulles to Los Angeles, conserving fuel by not using the usual Boeing 707 aircraft used for Air Force One.

After years of focused work to bring about deregulation, the 1978 Airline Deregulation Act became law, and United Airlines became a proponent of deregulation.

In May 1981, United Airlines launched its Mileage Plus program, shortly after the introduction of American Airlines' AAdvantage, marking the beginning of the modern frequent flyer programs.

In 1982, United Airlines became the launch carrier for the Boeing 767-200, receiving its first 767-200s on August 19. This marked a significant milestone in commercial aviation, with United's order being the largest at that time.

In 1983, United began flights to Tokyo from Portland and Seattle, gaining its first overseas route.

In 1984, United Airlines achieved a major milestone by becoming the first airline to serve all 50 states with commercial airports, demonstrating its extensive reach and network within the United States.

On May 17, 1985, United's pilots went on a 29-day strike, leading to significant labor unrest and disputes over pilot pay rates, ultimately impacting the airline's operations and management.

In February 1987, Ferris changed United's parent company's name from UAL Corporation to Allegis, but the name change was short-lived. Following Ferris' termination by the board, Allegis divested its non-airline properties in 1987 and reverted to the UAL Corp. name in May 1988.

In 1988, United flew a two-stop around-the-world flight to raise money for the Friendship Foundation using a Boeing 747SP-21 purchased from Pan American World Airways. The flight made a very short-lived record for the fastest flight around the globe; within a month, a Gulfstream IV business jet had broken Friendship One's record.

In 1989, United ordered the then-new Boeing 747-400.

The decline of Pan American World Airways offered opportunities; in 1991, United purchased Pan Am's routes to London Heathrow Airport. In direct negotiations with the British government, United also obtained rights to fly to Heathrow from Chicago. However, the aftermath of the Gulf War and competition from low-cost carriers led to losses of US$332 million in 1991 and US$957 million in 1992.

In 1992, United purchased Pan Am's Latin American and Caribbean routes and Miami gates, but allowed months to elapse between Pan Am's demise and its launch of service.

In 1993, United Airlines completely revised its branding and livery, introducing a primarily grey and dark blue fuselage with blue stripes on the tail. This 'Battleship' livery aimed to project a more business-like, global image for the rapidly expanding airline.

In 1994, United's pilots, machinists, bag handlers, and non-contract employees agreed to acquire 55% of company stock in exchange for 15% to 25% salary concessions. The Employee Stock Ownership Plan (ESOP) made United the largest employee-owned corporation in the world. United used the opportunity to create the low-cost subsidiary Shuttle by United in an attempt to compete with low-cost carriers.

On October 19, 1995, United Airlines Flight 976 experienced a disruptive incident involving an intoxicated investment banker who became disruptive, defecated in the aircraft, and created a security risk. The incident led to the arrest of the disruptive passenger and was later dubbed the worst ever case of air rage.

The 'Fly the Friendly Skies' slogan, introduced in 1965, was retired in 1996.

In 1997, United co-founded the Star Alliance with Air Canada, Lufthansa, Scandinavian Airlines, and Thai Airways.

In May 2000, United Airlines faced a bitter contract dispute with its pilots' union over pay restoration and concessions that were taken to fund the ESOP. The pilots' refusal to work overtime led to the cancellation of numerous flights during the busy summer season.

During the September 11 attacks, two of the four planes hijacked were United Airlines planes. One was flown into the South Tower of the World Trade Center, and the other crashed in rural Pennsylvania after passengers fought back against the hijackers.

In December 2002, UAL Corporation filed for chapter 11 bankruptcy protection after failing to secure a federal loan guarantee and facing financial crisis triggered by the 9/11 attacks and economic slowdown. This led to drastic cost-cutting measures and workforce reduction.

On November 12, 2003, United Airlines launched Ted, a new low-cost carrier aimed at competing with other low-cost airlines in the market.

United Airlines introduced the 'Rising Blue' livery on February 18, 2004, as a fresh start after emerging from bankruptcy protection. The new livery featured a white upper fuselage, lighter shade of blue for the aircraft belly, tail, and engines, and a series of progressively lighter blue lateral stripes.

On December 9, 2004, United Airlines made history by landing a Boeing 747-400 in Ho Chi Minh City, Vietnam, marking the first flight by a U.S. airline to the city since the end of the Vietnam War.

United Airlines raised $3 billion in exit financing and filed its Plan of Reorganization on September 7, 2005, as part of its efforts to exit bankruptcy protection.

On February 1, 2006, United Airlines emerged from Chapter 11 bankruptcy protection, marking the end of the largest and longest airline bankruptcy case in the history of the industry.

CFO Jake Brace expressed the company's intention to merge with a suitable partner. Bank of America and Fidelity Investments acquired a significant stake in the company. Pardus Capital Management LP urged for a merger between Delta and United.

UAL Corporation and US Airways Group were in advanced stages of merger negotiations. United pilots strongly opposed the proposal, and Lufthansa CEO expressed support for a merger between United and US Airways.

United Airlines announced the closure of its Ted unit and the reconfiguration for a return to mainline configuration to compensate for the retirement of Boeing 737s.

United Airlines announced a $15 fee for the first checked bag, becoming the second U.S. airline to do so after American Airlines.

United Airlines announced the cessation of several international routes, including San Francisco to Nagoya and Chicago to Mexico City.

On September 8, 2008, the price of UAL shares fell by nearly 99% in fifteen minutes to $0.01 US amid rumors of another bankruptcy, before NASDAQ temporarily halted trading. The rumors were traced to an old story on the South Florida Sun-Sentinel website about the 2002 bankruptcy being picked up by Google News and subsequently presented by Bloomberg LP as a breaking story. The share price subsequently recovered most of its value.

In January 2009, United announced a code-sharing agreement with Aer Lingus for flights between Washington Dulles and Madrid. Aer Lingus would operate the service, which is permitted under recent open skies agreements between the United States and Europe.

In April 2009, United eliminated the ability to call customer service, leaving reservations agents as the only reachable contact for the airline.

In May 2009, the U.S. Department of Transportation rated UAL eleventh among 19 US carriers in lost, damaged, delayed, or pilfered baggage, with 3.67 complaints per 1,000 passengers.

In June 2009, United asked manufacturers Boeing and Airbus to submit proposals to sell the airline up to 150 jets in a winner-take-all competition. United took advantage of declining sales at both plane makers to reap steep price reductions; the large size of this prospective order would also influence pricing.

In July 2009, the viral music video 'United Breaks Guitars' was released about a disputed damaged baggage claim with the airline. United said it wanted to use the video as a staff training tool to help the company improve its internal 'corporate culture' relating to its customer relations in that area of its services.

On April 16, 2010, United resumed merger talks with Continental Airlines. The board of directors of both Continental and UAL Corporation reached an agreement to combine operations to create the world's largest airline on May 2, 2010.

On May 3, 2010, it was announced that United and Continental Airlines would merge. The combined airline took the United name but used the Continental Airlines 'globe' identity and livery, designed in 1991 by the Lippincott company.

The merger of United and Continental Airlines was approved in September 2010, and the two companies merged on October 1, 2010.

On August 23, 2011, United Continental Holdings, Inc. announced the conversion to paperless flight decks by deploying 11,000 iPads to all United and Continental pilots. This initiative aimed to replace paper operating manuals, navigation charts, and other materials with iPads, resulting in reduced paper use, printing, and fuel consumption.

On November 7, 2011, United flew the world's first commercial aviation flight using a mix of Solajet, an algae-derived renewable jet fuel, and petroleum-derived jet fuel. This flight was operated by the Eco-Skies Boeing 737-800 aircraft, marking a significant milestone in the use of sustainable biofuels in commercial aviation.

United and Continental received a single operating certificate from the FAA on November 30, 2011, marking a significant milestone in their merger. The merged airline would use Continental's single operating certificate (SOC) under the 'United' name.

On March 3, 2012, United and Continental merged their passenger service systems, frequent-flier programs, and websites, virtually eliminating the Continental brand.

On May 31, 2012, United opened its operations center, which occupied twelve floors at the Willis Tower in the Chicago Loop.

The 'Proud to fly Team USA for over 30 years' campaign was used during the 2012 Summer Olympics to promote United Airlines as the official airline of the U.S. Olympic Team for more than 30 years. It featured voice overs by Matt Damon and aimed to showcase the airline's support for Team USA.

On December 15, 2012, a new Joint Collective Bargaining Agreement was ratified by a majority of the United/Continental pilots, which eliminated a scope clause preventing Continental from outsourcing the flying of regional jets with 70 or more passenger seats.

On January 15, 2013, United agreed to replace the Blended Winglets on its Boeing Next Generation 737 aircraft with APB's Split Scimitar Winglet (SSW), which was estimated to save the airline more than $250 million annually in fuel costs. This initiative aimed to significantly reduce drag and improve fuel efficiency.

On March 31, 2013, United and Continental merged into a single airline, with Continental becoming the surviving corporate entity and changing its name to United Airlines, Inc. The decision to retain the United name was aimed at preserving the brand equity of pre-merger legacy United Airlines.

The 'Fly the Friendly Skies' slogan, which was retired in 1996, was revived on September 20, 2013.

On June 2, 2015, all United Airlines flights were grounded. This significant event disrupted the airline's operations and caused inconvenience to many passengers.

On June 30, 2015, United invested US$30 million in Fulcrum BioEnergy, an alternative fuel company, to develop sustainable aviation alternative fuel refineries near its U.S. hubs. This investment aimed to produce over 180 million U.S. gallons of sustainable aviation alternative fuel per year, marking a significant commitment to sustainable aviation fuels.

United Airlines awarded airline miles as 'bug bounties' to hackers who identified gaps in the carrier's web security. Two hackers were rewarded with 1 million miles of air travel as of July 15, 2015.

On March 11, 2016, United became the first airline to fly on commercial-scale quantities of sustainable fuels procured from AltAir Fuels. This initiative aimed to reduce carbon dioxide emissions by over 60 percent on a lifecycle basis when compared to traditional jet fuel, marking a significant milestone in sustainable aviation.

On April 9, 2017, a passenger named David Dao was forcibly removed from United Airlines flight 3411, leading to injuries and public backlash. The incident resulted in a lawsuit, an undisclosed settlement, and changes in United's executive compensation and customer satisfaction policies.

Effective March 20, 2018, United Airlines suspended the PetSafe program for cargo transport of pets, following plans to mark pet carriers in the passenger cabin with bright tags and legislation introduced to ban the placement of pets in overhead compartments.

United Airlines revealed an updated livery on April 24, 2019, featuring a larger 'United' typeface, a dark blue wavy cheat-line, and the engines, winglets, and tail painted in a new shade of blue. The globe on the tail was also enlarged and changed to a lighter shade of blue.

On June 27, 2019, the parent company's name changed from United Continental Holdings to United Airlines Holdings.

On September 2, 2020, United Airlines indicated in a new memo to its employees that they planned to cut 16,370 jobs in nearly a month. The planned involuntary cuts included flight attendants, pilots, management jobs, mechanics, and airport operations, among others.

On September 24, 2020, United Airlines announced the launch of a new COVID-19 testing program for passengers, starting from October 15 of the same year. Initially, the testing was available for passengers traveling to Hawaii from San Francisco International Airport.

United Airlines became the first major US airline to announce a vaccine mandate for all staff on August 6, 2021.

Days before the internal deadline of September 27, 2021, United Airlines announced that more than 97% of the US based staff were vaccinated.

In December 2022, United Airlines placed an order for 100 787s to replace the 767s and older 777-200s. The aim is for United Airlines to become the flag carrier of the United States.

Severe weather along the Eastern Seaboard on June 24, 2023, led to an operational crisis for United Airlines, causing delays, cancellations, and diversions affecting at least 150,000 passengers. The CEO initially blamed FAA understaffing, but the Secretary of Transportation pushed back on these claims.

As of January 2024, United operates flights to 238 domestic destinations and 118 international destinations in 48 countries or regions across all six inhabited continents.

United plans to add a personal touchscreen television at the back of each seat on all Airbus A320 family and Boeing 737 aircraft by the end of 2025.