The AC-130 Spectre/Spooky, a modified C-130 cargo plane with a 105mm howitzer in it, designed for air to ground warfare is getting old and expensive to maintain. The planes were used heavily in Vietnam, and more recently as part of the "War on Terrorism," against suspected terrorists.
The Air Force would like to replace it with the AC-27J Stinger II gunship, a modified C-27 which is an intratheater transport plane similar to, but smaller than a C-130, designed to be operated by the Army. The cargo version, also known as the joint cargo aircraft, has a capacity similar to that of a heavy transport helicopter (about half that of a C-130), but with more fuel efficiency, speed, range and reliability than a helicopter, while still able to operate from short field airstrips. It is also cheaper than the C-130. The primary weapon has not been determined, but a 25mm to 40mm cannon is contemplated, so it would have the ability to destroy a tank or truck. The proposal was previously discussed here.
No other military aircraft in the world has a slug thrower that delivers a shell as large of that of the AC-130. All other aircraft use some combination of "dumb bombs," "smart bombs" and missiles to deliver larger explosive packages.
Like the A-10 Warthog, both the AC-130 and the AC-27J are designed for relatively long, subsonic air to ground combat and patrol missions, at relatively low altitudes. But, the A-10 is primarily envisioned as a tank killer with heavier armament and more allowances for targets that shoot back, while the AC-130 and AC-27J are primarily designed to target a lightly armed enemy encampment or group of soldiers, and to provide fire support during infantry battles or sieges of friendly encampments. The slower speed and smaller armament of these planes suits them for missions where it is important to be able to identify targets up close and to limit collateral damage and friendly fire casualties.
The proposed armament is similar to that of the guns on an A-10, or an Apache AH-64 helicopter, but an AC-27J or AC-130 has more capacity to carry ammunition for a sustained engagement than an A-10, and greater speed and range than an Apache AH-64.
Supersonic fighter aircraft, like the F-15, F-16, F-18, F-22 and three planned version of the F-35 (the Air Force F-35A, the Marine F-35B, and the Navy F-35C), have less of an ability to linger over a small part of a battlefield, are designed to have significant air to air combat capabilities, and are designed to use heavier bombs and missiles that might not be appropriate in close quarters where allied troops are nearby.
Historically, the Air Force has been less than enthusiastic about close air support missions, and has been highly skeptical that it is ever necessary for aircraft to engage opponents at close range, rather than using guided weapons and sophisticated sensors from a greater distance. But, the shift of U.S. military engagements towards counterinsurgency actions in Afghanistan and Iraq has encouraged the Air Force to prioritize this kind of procurement so that it can remain relevant.