The United States Marine Corps first developed an aeronautical company during WWI, first being used in an anti-submarine capacity and later used for limited bombing against German targets in France. 1

Incorporating the use of airplanes into the U.S. Marine Corps doctrine occurred along side similar developments and incorporations of aircraft into the U.S. Army and U.S. Navy. These developments saw the creation of the United States Army Air Service and the United States Naval Air Forces respectively. ^(2) All branches of the U.S. military were using aircraft as the technology developed and incorporating their use into each branches tactics and combat doctrines. So prior to the creation of the U.S. Air Force all branches maintained their own separate fixed-air wings.

The creation of the U.S. Air Force saw the U.S. Army retain only a very limited amount of fixed-wing aircraft, whereas the U.S. Marine Corps retained control of theirs.^(3) The Marine Corps aviation capabilities were almost stripped from it's control and even the existence of the branch as an independent entity was called into question. The Marine Corps did keep it's fixed-wing aircraft and in the ensuing years strengthened it's control and increased the number of planes under the branch as the Korean War raged. ^(4)

So why did the U.S. Army lose it's fixed-wing aircraft while the U.S. Marine Corps kept theirs after the creation of the U.S. Air Force in 1947?

Before the creation of the U.S. Air Force there was a great deal of debate as to what branches required the needs of certain aircraft, or any aircraft at all and to what extent they required those aircraft to be under their control. There was a large push to role all aircraft under the umbrella of the new U.S. Air Force, in the end the U.S. Marine Corps as well as the U.S. Navy were allowed to keep aircraft deemed essential to their operations as outlined in the National Security Act of 1947. ^(5)

The Marine Corps wants separate aircraft to maintain independence and flexibility in it's operations. Of the six primary functions of the Marine Corps aviation units, Offensive Air Support, Anti-Air Warfare, Assault Support, Air Reconnaissance, Electronic Warfare, Control of Aircraft and Missiles, offensive air support specifically close air support is the most vital. ^(6) The primary mission of the U.S. Marine Corps aviation units is in the support of ground forces, as such the Marine Corps has expressed a need for a closely integrated air-wing that can support their operational doctrine, one focused on the support of ground troops and their need of close air support. ^(7)

In summary the Marine Corps has fixed-wing aircraft because as with all branches it started with them and as aircraft became a part of their tactics and operational doctrine they saw a need to keep them after the creation of the U.S. Air Force in support of their specific goals, primarily goals focused on the support of it's ground troops.

As an addendum, albeit uncited, the role of the fixed-wing aviation units could be performed by the Navy and Air Force and undoubtedly both those branches have supported Marine ground forces, however the U.S. can afford to have separate branches with specialized air wings. Lastly there is a long history in the U.S. armed forces of each branch fighting for control over it's interests and 'share' of budgets and operational importance, viewed with this history it is unsurprising that a branch would push for increased operational independence within a given area that it saw as critical to it's role within the armed forces of the U.S.

Sources

^(1)Sherrod, Robert. "Marine Corps Aviation: The Early Days." Marine Corps Aviation: The Early Days, May 1952. Accessed August 11, 2018. r/https://www.mca-marines.org/gazette/marine-corps-aviation-early-days.

^(2) Sitz, W. H. A History of U.S. Naval Aviation. Washington: G.P.O., 1930.

^(3) "ARMY FIXED WING AIRCRAFT." www.army.mil. Accessed August 11, 2018. r/https://www.army.mil/article/137612/army_fixed_wing_aircraft.

^(4) Beauchamp, Bill. "The Challenge Of The Post-World War II Era: The Marine Corps, 1945-1957." Master's thesis, Marine Corps Combat Development Command, 1989. Accessed August 11, 2018. r/https://www.globalsecurity.org/military/library/report/1989/BBR.htm.

^(5) Futrell, Robert Frank. Ideas, Concepts, Doctrine: Volume I: Basic Thinking in the United States Air Force 1907-1960. S.l.: Air University Press, 1989.

^(6) Aviation Operations. Washington, DC: Headquarters, U.S. Marine Corps, 2000. r/https://www.trngcmd.marines.mil/Portals/207/Docs/TBS/MCWP 3-2 Aviation Operations.pdf?ver=2015-02-27-133614-823.

^(7) Allison, Fred, Major. "Lessons Learned: The USMC Approach to Close Air Support in Fallujah (Part One)." Marine Corps Gazette, January 31, 2010. Accessed August 11, 2018. r/https://sldinfo.com/2010/01/lessons-learned-corner-the-usmc-approach-to-close-air-support-in-fallujah-part-one/.