Weren't most runways of the WWII era dirt strips? I remember reading something, admittedly long ago, about how even in Britain concrete runways were a rarity simply because repairs were so much easier on dirt runways. This might have just been during the Battle of Britain though. I don't see flying something like a B-24 off of a dirt runway as being very efficient, especially considering how much rain the UK gets.
Either way, the true constraint would indeed be the base infrastructure rather than available runway space. Landing heavy bombers in a jungle clearing with a few tents and a hastily built radio tower is not impossible, but that isn't, and shouldn't be, an ideal place to support a round the clock bombing campaign.
What the game should do is have a refitting period between air missions during which the planes cannot take off again. Larger airbases would allow for heavier planes to be re-equiped faster, as well as allowing for more wings to operate in the same province without bogging things down. I guess it's similar to the frontage concept they are implementing for land units, only the capicity can be increased through building. Basically a level 1 airbase wouldn't allow for much more than a single wing of fighters to operate at full efficiency, but a fully developed base (or probably network of bases at that point) could keep several wings of heavy bombers going around the clock.