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I found an event in the files which create air units.

event = {
id = 655003
random = no
country = FIN

name = "The Finnish airforce grows!"
desc = "Our friends in Berlin have agreed to our proposal, and transfered our new interceptors to Helsinki! While costly, our airforce has gained tremendous strength!"
style = 0
picture = "itaplanes"

action_a = {
name = "Long live Finland, proud and strong!"
command = { type = relation which = FIN value = 20 }
command = { type = add_division which = "Lentorykmentti 2" value = interceptor when = 2 }
command = { type = add_division which = "Lentorykmentti 3" value = interceptor when = 2 }
command = { type = metalpool value = -1000 }
command = { type = energypool value = -800 }
command = { type = supplies value = -800 }
command = { type = money value = -120 }
command = { type = rarematerialspool value = -200 }
}

Just change the tag, names, id, and remove some commands. You now have an event giving you aircraft.
 
There's a folder with a bunch of txt files that list models. Use the name for the text files. Mountain had a really weird one.
 
Code:
event = {
	id = 999999
	country = XXX
	style = 0
	picture = air_reinforcements
	random = no

	name = "Airforce"
	desc = "Frikkin Planes, out tha sky!"

	action_a = {
		name = "How excellent!"
		command = { type = add_corps which = "Nyeeeeoooowwww"value = air where = -1 }
		command = { type = add_division which = "Pow Pow"value = interceptor when = 1 }
		command = { type = add_division which = "Dakka Dakka"value = interceptor when = 1 }
		command = { type = add_division which = "Bang Bang"value = tactical when = 1 }
		command = { type = add_division which = "Boom Boom"value = tactical when = 1 }
	}

Replace XXX with your country tag, and add to the end of any existing event file. Fire manually using f12 then 'event 999999'