The Siamese "Army Aviation Unit" was formed in a rice field south of Bangkok in 1913, with eight aircraft (4 Nieuport monoplanes & 4 Breguet biplanes). It relocated to the first airbase at Don Muang on 17 March 1914, and on 27 March 1914 was promoted to become the Army Air Corps, then the Army Air Division in 1918. (A number of personnel were sent to France in 1918, for training and combat service, but war ended before they saw action).
In December 1921, the Division (which included three wings) was removed from Army control, and placed directly under the Ministry of War as the Air Division. The name was changed again in 1935 to the Air Force Division, and two years later to the Royal Thai Air Force. At this point it consisted of five wings.
When Marshal Songgram began his military build-up in Thailand, the Royal Thai Air Force and Royal Thai Naval Air Force were equiped with obsolescent or obsolete aircraft, mainly of WW1 French designs. These included Breguet XIV and Nieuport XVII models. There had been some purchase of more modern aircraft in small numbers for evaluation purposes (such as the two Bristol Bulldogs acquired in 1927, and two Boeing P-12Es in 1931). By the late 30s, Thailand could boast an air fleet of 290 airplanes, but the vast majority were virtually useless in modern warfare. The best of these were 37 Curtis Hawk III fighter-bombers, 72 Vought SU-2 Corsair fighters, 6 Martin 139-W Bombers (commercial version of B-10) and 12-25 Curtis "Simplified" Hawk 75N fighters (with 20mm Madsen guns in underwing fairings, or .303" and .50" Vickers MGs). However, in the run-up to WW2 a new source of military material presented itself - Imperial Japan.
The Japanese wished to encourage Thailand in its nationalistic stance, and sold 93 modern aircraft to their new friends. These included 9 Tachikawa Ki-55 trainers, possibly 12 Nakajima Ki-43 Hayabusa "Oscar" fighter-bombers, 9 Mitsubishi Ki-30 "Ann" light-bombers, 9 Mitsubishi Ki-21-I "Nagoya"/"Sally" heavy bombers, 12 Nakajima Ki-27a "Nate" fighters, together with numbers of Ki-79a Mansyu trainers, Mitsubishi F1M2 "Pete" ASW aircraft to the RTNAF, and Tachikawa Ki-9 "Spruce" trainers/reconnaissance (RTNAF).
The most highly decorated pilots of the RTAF were those of the Corsair and Hawk III squadrons, including Squadron Leader Sanit Nunmance (an SU-2 pilot), the most decorated Thai airman of the war.
The following were cadged from a (Swedish?) site which I can no longer find. They show aircraft on display at the RTAF museum at Don Muang.
Breguet XIVB:
Tachikawa Ki36 Trainer:
Nieuport 17(?):
Vought Corsair V-93S:
"Demon" insignia from above:
The only action which I have any detail for is the RTAF attack on 11 January 1941. In this, the 6 Martin 139-Ws were sent to bomb Hanoi, escorted by 4 Curtiss Hawk-75Ns. They were intercepted by 4 Moraine-Saulnier 406s (from Escadrille 2/595?). The Thais claimed 2 MS406 shot down, which the French refuted (2 of E.C. 2/595's aircraft were out of service at the end of the conflict - perhaps they were damaged in the combat and claimed as kills by the Thai pilots?). The Thais tended to carry out daylight bombing missions on Cambodia and Laos using their Martin bombers (and perhaps the "Sally"s, though unlikely to have been received in time), while the French bombed Thai towns at night using their Farman 221 heavy bombers or Potez 542 light-bombers (both of which were night-capable).
The RTAF OB for December 1940 according to Shores and Ehrengardt (L'aviation de Vichy au Combat, Vol.1: Les Campagnes Oubliées, Christian-Jacques EHRENGARDT and Christopher SHORES, Paris : Lavauzelle, 1983).
Northern Region Command
#73 Mixed Wing
Observation Squadron # 32 (Ubon), 9 Vought V-93S
Bomber Squadron # 50 (Ubon), 6 Martin 139WS
#35 Mixed Wing
Observation Squadron # 34 (Udorn), 9 Vought V-93S
Pursuit Squadron # 50 (Nakhon Panom), 9 Curtiss Hawk III
Southern Region Command
#66 Wing
Phibun (Phibul?) Songkram Squadron # 1 (Don Muang - Bangkok), 12 Mitsubishi Ki.30 "Ann"
Phibun (Phibul?) Songkram Squadron # 2 (Don Muang - Bangkok), 12 Mitsubishi Ki.30 "Ann"
Pursuit Squadron # 60 (Don Muang - Bangkok), 11 Curtiss Hawk 75N
#74 Mixed Wing
Observation Squadron # 44 (Chantaburi), 9 Vought V-93S
Pursuit Squadron # 71 (Chantaburi), 9 Curtiss Hawk III
Pursuit Squadron # 72 (Chantaburi), 9 Curtiss Hawk III
Observation Section (Chantaburi)
#75 Mixed Wing
Pursuit Squadron # 73 (Sisaket), 9 Curtiss Hawk III
Pursuit Squadron # 80 (Prachinburi), 9 Curtiss Hawk II
Attack Squadron # 35 (Prachinburi), 9 Vought V-93S
Notes :
This OB gives only 11 Hawk 75Ns because Thailand only bought twelve with 2 fixed 8mm Vickers fuselage guns and removable wing pods for a 20mm Madsen cannon or a Vickers 8mm MG. One was lost in an accident piror to the FTW, one was destroyed on the ground by French night bombers and one was lost in a collision with a Hawk III.
I assume that the "Phibun Songkram" were named after the Thai leader as he had bought these new Japanese aircraft?
The numbers of aircraft do not quite tally with my comments above, but are generally in line with other sources. Obviously, it does not cover the RTNAF, which may account for some of the discrepancies.
On December 8th 1941, "Wing 5" of the RTAF apparently defended their airbases in southern Thailand against the Japanese invasion, fighting as infantry in the "Battle of Prachuap Bay". I think that this would be the 75th Mixed Wing above?