With predictions like H.G. Wells in Shape of Things to Come of bombers using high explosive, incendiary, chemical and possibly biological weapons to devastate civilisations and the bomber (ICBM) will always get through it is understandable there were 1930's worries about "bomber gaps", credibility of deterrence and only one way to fight/win the next war. Then radar appeared and rather remarkably chemical weapons remained unused.
The pre war it was the USN and IJN, not the USAAF or IJA, that invested in dive bombing, you only need to look at the number of US dive bomber designs ordered and built versus torpedo bombers and the Luftwaffe picked up on that, deciding the greater accuracy was worthwhile. Dive bombers were generally slower as drag is good for a dive bomber and had generally smaller bomb loads over generally shorter ranges given the weight required for the necessary airframe strength. That lead to them being rapidly phased out in New Guinea since airfield space was tight. The RN decision that the main anti shipping weapon would be the torpedo played a part in Britain's lack of dedicated dive bomber designs, versus requirements its bombers be able to attack at various dive angles, including the 4 engined types, along with exaggerated ideas of how accurate these bombers would be. The crisis of 1940 forced rationalisation of the number of types of aircraft in and going to be in production. The attributed successes of the Ju87 in France meant dive bomber orders were placed in the US while the RAF was confident it had proved how vulnerable dive bombers were when intercepted and that they could be regularly intercepted. The Germans agreed.
As a thought in WWII the RAF regiment ended up with effectively infantry and armoured car units, I wonder what the army would have made of such a request pre war.
When it comes to army support it evolved and required the army to agree. Pre WWII the RAF was not interested in providing the army's idea of ground support. In any case it took until 1938 for the British to decide on a 2 division expeditionary force if war came again to Europe and 1939 to upgrade the plan to something like the 1940 BEF. The RAF then supplied about as many reconnaissance and bomber aircraft per division in 1940 as it later did for 21st Army Group but nowhere near as many fighters.
The 1918 combined arms offensive, infantry, artillery, armour and airpower were known to be effective but costly to aircraft, given they were operating over the front line the airpower direct effects were reduced as the enemy were generally dug in, and the risk of friendly fire was high, but keeping the enemy forces pinned down or having to look all around helped the attacking ground forces, even though the aircraft had poor communications with the attacking ground forces. Force the enemy to move and WWI airpower could cause considerable losses.
WWI was largely a stalemate on the western front as the defender could normally rail in troops fast enough to plug any gaps before they became large breeches. In the 1920's and 1930's airpower could finally present a serious threat to rail systems, trains, tracks, bridges etc. In simple terms the RAF turned to the interdiction model as the support method under air force control and that can be seen in 1940 by both the forces in France and by Bomber Command, the army stayed with battlefield support, plus an air umbrella to keep the enemy air force away, with aircraft under army control. Neither force was really talking about the sort of combined HQ system needed to make anything work effectively, becoming stuck on who would command. Neither side had much data on what weapons and tactics were best or how effective they would be and did few exercises to find out, think the air force over estimating bomb damage and the army at times underestimating air power effects on it while overestimating it on the inferior enemy.
At this point I wonder what the army would have done had the air force insisted on army artillery doctrine being a form of umbrella, where the divisional artillery units are broken up so each platoon and maybe company HQ has its own artillery piece, under the infantry commander's control. It would certainly provide near instant fire response requests. Incidentally the British army doctrine that evolved during WWII of matching specific artillery units to specific infantry ones at least within the division was a more centralised version of this, together with forward observers with relevant authority to order shoots. It gave speed, firepower and mutual understanding. In France in 1940 artillery units would report they had targets but by the time permission to fire had been granted the targets had moved away. Part of the changes from the army side were in 1940 the average British infantry division had 75 radios, most Morse, versus nearly 1,000 in 1944, despite having around the same firepower in 1944 as 1940.
Looking at the USAAF doctrine and ideas in the 1920's at least, there is an interesting contradiction, the ideas were what to do in a major war, but they were often discussing allocations of small numbers of aircraft, flights to almost individual aircraft, the reality of what they actually had intruding on the ideas of what they would have. It shows up a main problem, figuring out how many aircraft of what type in the ground support units per ground unit, as well as figuring out how many ground units which meant figuring out what sort of war and where.
Someone can no doubt supply more exact definitions but divisional commands generally worry about the terrain in front and behind them to a depth of a few times artillery range, as you go up the chain of command the depths increase. Adding airpower to the ground HQ responsibilities means a major increase in depth to manage given the range of friendly and enemy aircraft, unless there is a second air force looking after deeper defence and offence, essentially having both a tactical and strategic air force, again not a topic pre war. Whoever had the aircraft under their control needed to be able to handle operations hundreds of miles from the front line if only to guard and attack supply lines.
The 1941/42 Western Desert fighting provided the incubator for allied ground support doctrine but even that had limits, few fixed targets, like rail, road, bridges, even airfields to an extent. The importance of suppressing the enemy air force before the ground battle was realised, similar really to the pre battle suppressing of hostile artillery. The concept of light/medium bombers on standby, awaiting an attack order proved highly wasteful, there might not be a target and even if there was it took hours between the target being identified and the aircraft attacking. Better to give those units a specific target, relying on the more numerous fighter bombers (which was in many ways a reprise of 1918) for operations closer to the front line. Though during Operation Crusader there were multiple front lines often many miles apart. Even using the fighters could have significant delays and there were the friendly fire incidents, it did not help both sides were using each other's vehicles in numbers.
Pre war ground support ideas had no fighter bombers, instead using multi crew bombers and ground attack designs which were more vulnerable to hostile fighters, there was also no radar, so warnings required either scout aircraft with reliable "long range" radios or observers as the enemy crossed the front line. The air umbrella idea was actually done by the allies in Normandy with direct radio communication between ground and air (The Germans used that in 1940) along with a tasking system of the level bombers and some fighter bombers doing interdiction and small formations of fighter bombers often assigned to a specific ground controller for a fixed time then released to either return or roam a given area for targets. Flying lots of small formations of heavily laden aircraft meant timely airpower intervention, it was also a scenario for major losses if the enemy air force was active. It was possible because of how big the allied air forces were versus the Luftwaffe, not only the tactical air forces but Fighter Command for the deeper rear areas and the strategic forces for the deeper penetrations into hostile airspace. None of the pre war ideas talked about how big the air force needed to become to do this. It is sort of like the USN versus IJN pre WWII war plans, each ended up with a negotiated peace, not only through convention but because of the calculated shipping etc. needs for an invasion across the Pacific were way more than anyone thought possible to build. As it was in 1945 the US needed to load ships on the US east coast for the planned invasion as the trans continental rail links could not handle the load.
I am sure the pre war RAF would have supported the numbers required for umbrellas, it meant in March 1944 the air force had about 1.2 million personnel to the army's 2.86 million to 835,000 RN. Would the army and navy? In September 1939 it was 180,000 Navy, 897,000 Army, 193,000 Air Force.
Even with all the control and airpower some US servicemen apparently called the 9th Air Force America's Luftwaffe and during the Normandy fighting one unit complained about a devastating air raid, the complaint ended up causing such a fuss Bradley and also Quesada went to see what had happened for themselves. Turned out around a squadron of Bf109s had done a couple of strafing runs, causing very little damage. Bradley went back and wrote his memo about no such thing as 100% air cover and units should take this into account. Then again there are the photographs Quesada sent to Bradley, on the allied side people moving around all day in close proximity, dumps with near zero camouflage, the front line easily seen as suddenly it became a deserted landscape, no people, no vehicles. Makes things easier for the army.
Still allied airpower in 1944 worked much better at interdiction including suppressing and attacking road transport, denying the Germans day and at least partly night mobility, than hitting dug in troops, the front line operations required immediate ground follow up to be effective and had the highest risk of friendly fire.
As of 1939 neither the British Army nor the RAF had worked out a ground support doctrine or system fit for purpose, starting with the combined HQ or at least staff system necessary to even try, beyond at best listing transport targets to attack to cut supply lines. Neither was all that interested in the other's ideas. Yet it must be pointed out within both there were people working on co-operation with limited resources. Counting 1 Group (10 squadrons of Battles) as part of Bomber Command but the fighter and Blenheim squadrons in France as army support, in September 1939 Bomber Command had 39 Squadrons, Fighter Command 38 Squadrons, Coastal Command 19 Squadrons and 22 Group (Army Co-Operation) 13 squadrons (not all in France though). There were those within the RAF who were still thinking of the France based Battles bombing the Ruhr, at least in 1939, but also a new 1 Group began to be formed in Britain as the Battles became more orientated towards army support missions.