The Army introduced the A designation category in 1926. It applied to attack and light bombardment categories of combat aircraft.
This is the original A-series, which covered the years 1926-1948:
A-1 Not assigned, since it clashed with th Cox-Klemm XA-1, an ambulance plane still in service. Douglas XA-2 Conversion of O-2 two-seat observation biplane to attack configuration. Lost out to Curtiss A-3 in attack plane competition. Only one built. Curtiss A-3 Falcon O-1B observation plane adapted to attack role by addition of bomb racks and additional 0.30 cal gun in each lower wing. 144 built. Curtiss XA-4 Falcon A-3 modified to test 440hp P & W R-1340 Wasp radial engine. Only one built. Curtiss XA-5 Attack counterpart of XO-16 two-seat observation biplane. Canceled before any could be delivered. Curtiss XA-6 Attack counterpart of XO-18 two-seat observation biplane. Cancelled before any could be delivered. General Aviation Two-seat low-wing monoplane ground attack plane. (Fokker) XA-7 Lost out to Curtiss A-8 for production orders. Curtiss YA-8 Two-seat monoplane ground attack aircraft. One 8 built. Lockheed YA-9 Two-seat monoplane attack aircraft. Attack version of Lockheed YP-24 experimental pursuit aircraft. Parent company (Detroit Aircraft) went belly-up and none were produced. Curtiss YA-10 First YA-8 reequipped with a 625 hp P&W Hornet radial engine. 175 mph. Proved that radial engine was preferable to liquid-cooled engine for attack role, and convinced Army to have A-8B aircraft on order be produced as A-12. Consolidated A-11 Development of Lockheed YA-9. Two-seat monoplane attack aircraft. Performance was well advanced over its contemporaries, but the Army disliked liquid-cooled engines for ground attack planes. Pursuit versions were P-30 and PB-2A. Curtiss A-12 Shrike Two-seat monoplane attack aircraft. Adaptation of A-8 design to 690 hp Wright R-1820 Cyclone radial engine. Forty-six A-12s built for U. S. Army. 20 export versions sent to China. Northrop XA-13 Two-seat monoplane attack aircraft. Extensively modified version of Gamma 2C commercial monoplane. Curtiss XA-14 Two-seat, twin-engined ground attack aircraft. Only one built. Martin XA-15 Attack version of Martin YB-10 bomber. Abandoned in design stage in favor of Curtiss XA-14. Northrop XA-16 XA-13 fitted with 950 hp P&W R-1830-7 radial. Only one built. Test results indicated that the aircraft was overpowered and that production aircraft should have either a smaller engine or larger tail. Northrop A-17 Two-seat, single-engine attack aircraft. One P&W R-1535 radial engine. A-17 had fixed landing gear, A-17A had retractable landing gear. Numerous exports. Curtiss Y1A-18 Shrike Two-seat, twin-engined ground attack aircraft. Same armament as XA-1, but bombs were carried in wing bays rather than in fuselage. 13 built. Used primarily for operational training. Vultee YA-19 Two-seat attack aircraft. Evolved from V-11GB export attack aircraft. Five built. Douglas A-20 Havoc Twin engine, three-seat attack bomber Most widely-used aircraft in the A series. Produced in many different versions with many different armament schemes. 7478 built. F-3 was photo recon version. P-70 was night-fighter adaptation equipped with radar. Stearman XA-21 Three-seat, twin-engine light bomber. No production orders. Only one built. Martin A-22 Maryland Twin engine, three-seat attack aircraft. Lost out to Douglas A-20 for Army production orders, but ordered by French. Flew in combat during German invasion. After French collapse, remaining Marylands were taken over by British. Served with British units in Mediterranean and North Africa. Some service with Vichy French. Martin A-23 Proposal for twin-engine attack aircraft powered by Wright R-3350 radials. Project was dropped. Douglas A-24 Dauntless Army version of SDB Dauntless carrier-based dive bomber. A-24 was similar to Navy SBD except for removal of deck landing gear and a new tail wheel. 953 built. First examples entered Army service in 1941. Most A-24s remained Stateside, where they were used primarily for training, but some did see combat. Curtiss A-25 Shrike Army version of SB2C-1 Helldiver carrier- based dive bomber. 900 delivered to US Army. Army eventually decided that it didn't need dive bombers, and A-25 never entered combat. Most used as trainers and target tugs. Ten were delivered to Australia. 410 were turned over to the US Marine Corps, which used them as operational trainers under the designation SB2C-1A. Douglas A-26 Invader Three-seat, twin-engine light bomber. Regarded as USAAF's best twin-engine bomber at end of WW2, and plans were under way to convert all A-20, B-25, and B-26 units to the A-26 at war's end. B-26 saw lots of action in Korean War in armed reconnaissance and interdiction role. Withdrawn from front-line service with USAF in mid-1950s and replaced by B-57 and R/B-66. Many Invaders were sold as surplus on the civilian market, and converted to executive transports. Many were exported to foreign air forces. Some are still in service. North American A-27 Designation for 10 NA-69 export attack planes ordered by Thailand, but siezed by Army lest they fall into Japanese hands. Used only for training. Lockheed A-28 Hudson Hudson was military adaptation of Model 14 commercial airliner designed to British requirements. Nearly 2000 Hudsons were acquired by British, either by direct purchase or via Lend-Lease. After Lend-Lease was approved in 1941, outstanding British contracts for Hudsons (and other aircraft as well) were taken over under Army contracts and produced under USAAF designations and serial numbers. Most went directly to British Commonwealth air forces, and never served with USAAF. Lockheed A-29 Hudson Designation given to Hudson light bomber ordered under Lend-Lease for service with British Commonwealth forces. Differed from A-28 in being powered by two Wright R-1820 Cyclone radials. Most went directly to Commonwealth air forces and never served with USAAF. Martin A-30 Baltimore Twin-engine attack bomber built for British use under Lend-Lease. Served exclusively in the Mediterranean area with British, South African, Greek and Italian Co-belligerent air forces. None used operationally by USAAF. 1575 built. Vultee A-31 Vengeance Two-seat, single-engined dive bomber. One Most were sent to the British under Lend-Lease. Vengeances in US service used primarily for training and never saw combat. Brewster XA-32 Single-seat attack bomber. Speed and range performance fell below expectation. Project cancelled. Douglas A-33 Designation for 31 Douglas 8A-5 attack planes taken over from Peruvian order. Used for general utility service. Never saw any combat. Brewster A-34 Bermuda Designation assigned for Lend-Lease documentation to SB2A Buccaneer naval dive bomber. None ever served with AAF. Vultee A-35 Vengeance Two-seat, single-engined dive bomber. Modification of A-31 with four fixed 0.50 cal guns in wing, one 0.50 cal gun in rear cockpit. Most sent to the British and the Australians. Some given to Free French. Never saw combat in USAAF service, serving only in training and target-towing roles. North American A-36 Mustang Dive bomber version of P-51 Mustang fighter. First version of Mustang to see action in USAAF service. Saw action primarily on Italian front and in India. Hughes XA-37 Twin-boom, twin-engined light bomber project. Most of airframe constructed of Duramold, a material made of heat-bonded wood and plastic. Carried no armament. USAAF considered it as a bomber escort, as a fighter, and as an attack aircraft. In its fighter incarnation, it seems to have had the designation "XP-73" reserved for it. The designation "XA-37" refers to its attack incarnation. Beechcraft XA-38 Destroyer Two-seat attack bomber. Two Wright R-3350 radials. Delayed by lack of availability of engines, which were needed by B-29. Never reached production. Kaiser-Fleetwings XA-39 Single seat, single-engined light bomber. One P&W R-2800 radial. Never got past the mockup stage. Curtiss XA-40 Single seat, single-engined light bomber. One Wright R-3350 radial. Never got past the mockup stage. Convair XA-41 Single seat, single engine close-support aircraft. One P&W R-4360 radial. Four 37-mm cannon and four 0.50 cal guns in the wings. Internal bomb bay could carry 3000 lbs of bombs. Flight tests showed promise, but Army close support was well provided for by P-47 Thunderbolt and A-26 Invader. No production orders. Only one built. Douglas XA-42 Proposal for twin-engine attack aircraft powered by two Allison V-1710 liquid cooled engines mounted in fuselage and driving two contra- rotating props in tail. Design showed greater potential as a medium bomber and was redesignated XB-42 Curtiss XA-43 Proposal for two seat, four jet attack plane. Project cancelled in early design stage. Funds and serial numbers transferred to similarly- configured XP-87 Blackhawk night fighter. Convair XA-44 Tactical bomber with three 4000 lb. st. General Electric J-35 turbojets buried in the fuselage and fed by two lateral intakes. 30-degree swept-forward wing. Redesignated XB-53 in 1948. Cancelled before any could be built. Martin XA-45 Three-jet light tactical bomber. Redesignated XB-51 in 1948. [The original A series ends at this point.]
In 1948, the separate A category was eliminated from the Air Force designation scheme. Henceforth, all future Air Force planes that fell under the attack category were to be classified under B (for bomber). At that time, only two aircraft from the original attack series still remained in service with the Air Force--the Douglas A-26 Invader and the Douglas A-24 Dauntless. The Invader was redesignated B-26. There was no danger of confusion with the Martin B-26 Marauder, since all Marauders had been removed from active service by that time. So there were TWO airplanes that carried the B-26 designation, but they didn't both serve at the same time! In addition, the few Douglas A-24s still serving with the USAF in 1948 were redesignated F-24, a fighter category. Again, there was no possibility of confusion, because the prior P-24 designation had been carried by a Lockeed design of the early 1930s which never attained quantity production.
Prior to 1962, the US Navy had its own separate designation scheme for its attack aircraft. In 1962, Secretary of Defense Robert McNamara decided to unify the aircraft designation schemes of all the services under one umbrella. In addition, the A for attack category (which had been eliminated in 1948) was reintroduced. The Air Force initially had no aircraft that fell into the A category, but the Navy did have some and all of these were duly redesignated. In later years, the Air Force did acquire some attack aircraft, and these were assigned numbers in the A series in the sequence in which they were ordered into service.
The attack planes in the post-1962 A-category are:
Douglas A-1 Skyraider Formerly designated AD. Single-engine, carrier-based attack aircraft. One Wright R-3350 radial. 322 mph at 15,000 ft. Four 20-mm cannon in wings, underwing load of up to 10,000 pounds of bombs. Night-attack, antisubmarine warfare, ambulance, cargo, and radar picket versions built. Also served with USAF, Royal Navy, France, Vietnam. Served in Korean, Algerian, and Vietnam wars. Total of 3180 built. North American A-2 Savage Formerly designated AJ. Three-engined carrier- based strategic bomber. Two 2300 hp P&W R-2800-44W radials in underwing nacelles, one Allison J33-A-10 turbojet of 4600 lb. st in rear fuselage. Most Savages did not serve in their intended roles as strategic bombers, but were converted as flight-refuelling tankers. Douglas A-3 Skywarrior Formerly designated A3D. Twin-jet, swept-wing carrier-based strategic bombing aircraft. Two P&W J57-P-6A turbojets. Many converted to aerial tankers, electronic countermeasures planes, and trainers. Total of 284 built. Douglas A-4 Skyhawk Formerly designated A4D. Single-engine carrier- based attack aircraft. Versions A through D powered by Wright J-65 jet. E version powered by P & W J-52 jet. Extensive service with US Navy during Vietnam War. Served with IAF during "War of Attrition" and Yom Kippur War. Served with Argentina during Falklands/Malvinas campaign. Also delivered to New Zealand, Singapore, and Kuwait. 2960 built. North American A-5 Vigilante Formerly designated A3J. Twin-engine, two seat carrier-based supersonic strategic bomber and reconnaissance aircraft. In 1964, it was decided that Navy strategic role would in the future be pursued exclusively by Polaris-armed nuclear submarines and not by aircraft. Remaining A-5s were all converted to RA-5C reconnaissance configuration. Total of 156 Vigilantes built. Grumman A-6 Intruder Formerly designated A2F. Twin engine, two seat carrier-based all-weather attack aircraft. Two P&W J-52 turbojets mounted below wing roots. Ling/Temco/Vought A-7 Single seat attack and close support aircraft. Corsair II Looks much like a snub nose F-8 Crusader. A, B versions are Navy carrier-based attack planes powered by P&W TF-30 turbofans and armed with two 20 cannon. D is Air Force land- based version with one Allison TF-41 turbofan and armed with one 20-mm rotary cannon. McDonnell-Douglas AV-8 Single-seat V/STOL close support and tactical Harrier reconnaissance aircraft. License-built British Aerospace Harrier. Northrop A-9 Twin-engine, single-seat close air support aircraft. Two Lycoming YF102 turbofans under the wing roots of a high wing. Lost out to Fairchild Republic A-10 for production orders. Fairchild Republic A-10 Twin-engine, single-seat close air support Thunderbolt II aircraft. Two General Electric TF34 turbofans in pods above and behind low-mounted wings. Primary armament is seven-barrel GAU-8/A 30-mm antitank cannon. A-11 This one appears never to have been assigned. The reasons are obscure. One possibility is that the A-11 designation was not used because someone might "confuse" it with the "A-11" designation which LBJ applied deceptively to the supersecret Lockheed A-12 spyplane. Another possibility is that A-11 is the designation given to an as-yet-unannounced "black" project. Noone seems to know for sure. Anyone who knows isn't talking. McDonnell Douglas/General Two-seat low-observable medium attack aircraft. Dynamics A-12 Extensive use of composites. Designed as Avenger II replacement for Grumman A-6. Large flying wing. Typical A-6 weapons load internally. Additional ordinance can be carried externally when stealth is not important. Range and speed supposedly exceed those of A-6. Project was cancelled in early 1991 due to cost overruns and schedule slippages.
From this point on, there are "gaps", since these later A- aircraft were attack planes that had been converted from other roles.
General Dynamics A-16 Proposed close air support version of F-16 Fighting Falcon fighter. Not built. McDonnell Douglas A-18 Designation given to attack version of F-18 Hornet twin-engined, single-seat carrier-based fighter. Initially, the F-18 and A-18 were envisaged as separate and distinct aircraft fulfilling different requirements. However, during the design phase the two aircraft evolved in such a manner that they differed from each other only in minor details, and the Navy decided to combine both planes under the joint designation F/A-18. Douglas A-26A Counter Invader Originally designated B-26K. Conversion of existing B-26 airframe to counter-insurgency role. While operating in Thailand, the aircraft were redesignated A-26A, since a treaty between that government and the USA forbidded the basing of "bombers" in Thailand. Phased out of service in 1969. Cessna A-37 Dragonfly Initially designated YAT-37D. Attack version of T-37 tandem, two-seat primary jet trainer.