The airship should be like a gunship but be able to move across any terrain. In reality, an airship wouldn't need to go straight from airport to airport like a plane. Those things were even used for expeditions to the poles.
So I'd say make em a 3-square-per-turn-fly-anywhere-gunship with the added option of bombarding city defences.
Additionally, create a biplane that comes with flight, that can do the usual plane mission but that's rather weak - say 2 or 3 strength - but has intercept 10% and +75% to +100% strength against airships.
Also give the machine gun's a 10% intercept chance against planes, maybe even a strength bonus against airships.
That way we'd have MG, biplanes and airships together (as they were in actual history). The biplanes would be useful as air-to-air tools mainly. MG's would function as early AA's. And airships would be able to take their role as naval (and other) scouts, weak city bombers and crappy frontline bombers.