According to the National Security Act of 1947 (61 Stat. 502), which created the USAF:

In general the United States Air Force shall include aviation forces both combat and service not otherwise assigned. It shall be organized, trained, and equipped primarily for prompt and sustained offensive and defensive air operations. The Air Force shall be responsible for the preparation of the air forces necessary for the effective prosecution of war except as otherwise assigned and, in accordance with integrated joint mobilization plans, for the expansion of the peacetime components of the Air Force to meet the needs of war.

§8062 of Title 10 US Code defines the purpose of the USAF as:

• to preserve the peace and security, and provide for the defense, of the United States, the Territories, Commonwealths, and possessions, and any areas occupied by the United States;
• to support national policy;
• to implement national objectives;
• to overcome any nations responsible for aggressive acts that imperil the peace and security of the United States.

The stated mission of the USAF today is to "fly, fight, and win in air, space, and cyberspace".

Operational Functions

The Air Force describes its mission in terms of 17 operational functions:

Strategic Attack – offensive action that most directly achieves national security objectives by affecting the adversary’s leadership, conflict-sustaining resources and strategy.

Counter-air – operations to attain and maintain a desired degree of air superiority by the destruction, degradation, or disruption of enemy forces. Counter-air takes the form of both [[offensive counter-air]] against enemy air and missile power at its source, and defensive counter-air against attacking enemy air and missiles over friendly territory.

Counter-space – destruction, degradation or disruption of enemy space capability.

Counter-land – air and space operations against enemy land forces, including air interdiction to divert, disrupt, delay, or destroy the enemy’s surface military potential before it can be used effectively against friendly forces, and close air support to help friendly surface forces in contact with enemy forces.

Counter-sea – tasks including sea surveillance, anti-ship and anti-submarine warfare, aerial minelaying, and air refueling in support of naval campaigns.

Information Operations – actions taken to influence, affect, or defend information, systems, and/or decision-making, through influence, network warfare, and electronic warfare operations.

Combat Support – capabilities, functions, activities, and tasks necessary to create and sustain air and space forces, including the procurement, maintenance, distribution, and replacement of personnel and materiel.

Command and Control – exercise of authority and direction by a properly designated commander over assigned and attached forces in the accomplishment of the mission, including both process and systems.

Airlift – transportation of personnel and materiel through the air.

Air Refueling – in-flight transfer of fuel between tanker and receiver aircraft.

Space-lift – delivery of satellites, payloads and materiel to space.

Special Operations – airpower conducting unconventional warfare, direct action, special reconnaissance, counter-terrorism, foreign internal defense, psychological operations, and counter-proliferation.

Intelligence – product resulting from the collection, processing, integration, analysis, evaluation and interpretation of available information concerning foreign countries or areas.

Surveillance and Reconnaissance – systematic observation of air, space, surface, or subsurface areas, places, persons, or things, by visual, aural, electronic, photographic, or other means. Surveillance is a continuing process, not oriented to a specific target, while reconnaissance looks for specific information and generally has a time constraint.

Combat Search and Rescue – recovery of isolated personnel with rotary- and fixed-wing aircraft.

Navigation and Positioning – provision of accurate location and time of reference.

Weather Services – environmental information, including both space environment and atmospheric weather.

Search and rescue

The National Search and Rescue Plan designates the United States Coast Guard as the federal agency responsible for maritime search-and-rescue (SAR) operations, and the USAF as responsible for aeronautical SAR in the continental U.S. with the exception of Alaska. Both agencies maintain Joint Rescue Coordination Centers to coordinate this effort. To help the USAF with the vast number of search and rescue operations, the USAF assigns units of the Civil Air Patrol — the official U.S. Air Force Auxiliary — in over 91% of inland search and rescue missions.

Air sovereignty

The USAF, through the Air National Guard, is the lead agency to maintain control of America's airspace.

On July 30, 2009, Lt. Gen. Harry Wyatt, director of the Air National Guard said that "Technologies needed for the mission include an active, electronically scanned array radar (which can be used to detect small and stealthy air threats including cruise missiles), infrared search and track systems and beyond-line-of-sight communications".

On September 14, 2009, Gen. Norton A. Schwartz, chief of staff of the USAF, said that he hopes "to bring a combination of F-22, F-35, legacy aircraft, including upgraded F-15 and F-16 fighters, and unmanned aircraft to the [air sovereignty alert] ASA mission."

Even so, the USAF plans to retire up to 80% of their total force air sovereignty mission aircraft, which would leave no usable aircraft at 18 current air sovereignty sites after 2015. The GAO found that 17 of the 20 commanders of the ASA units "stated that the Air Force treats ASA operations as a temporary mission and has not provided sufficient resources."

The USAF has decided to accept "moderate risk" for the air sovereignty mission as well as deep strike and close air support, under optimistic assumptions for F-35 production.

The Defense Department has used USN and USMC aircraft for the Air Sovereignty Mission and may do so in the future.

Irregular warfare

In response to the conflicts in which the United States has been engaged since the end of the Cold War, on August 1, 2007, Air Force Doctrine Document 2-3 was released showing how air power could be used to support or defeat an insurgency.

To support these missions, the USAF considered outfitting a counter-insurgency wing with small, ground attack aircraft that can also be used for training USAF and allied pilots in addition to counterinsurgency operations. However the 2010 QDR shifted the future light fixed-wing aircraft to the Air Force’s 6th Special Operations Squadron to be used to train allied forces.

Airlift

The USAF provides both strategic and tactical airlift in support of wartime, peacetime, and humanitarian efforts of the Department of Defense.