I would just have them describe what it was like to be there. What they thought when they were in the air. What they thought when they weren't in battle. What memories of war are most vivid for them.

The details of units, planes, battles, etc. are easy to reconstruct from paper records. But the subjective experience is not, and this is where oral histories do the most good.

I would ask about what they thought of the morality of their mission. Did their command or chaplains address this issue?

Mainly, though, let them talk. Ask followup questions about interesting things they say. Listen so you don't miss an opportunity, then worry about what question you're going to ask next.

And follow all best practices for oral history.

(Edited to add) Ask them what they did for fun. I've read a number of stories of mad dashes to pubs, wild auto races either on public roads or on the airfields themselves, "requisitioned" alcohol or equipment. Some lighter moments would probably be a good idea.

Did they have free time? What did they do in their free time, if they had any?

I agree with restricteddata, that it is best to ask personal questions and to let them try convey their feelings. Did you feel pity for those that you killed? Did you visit places that you bombed after the war? Did you try to reconcile? How do you feel about Germans now?

What was it like to loose friends? Did you avoid personal relations with other pilots? Did you have friendships with girls? Did you like Engliand? Try to get the feeling of what it feels like to be in a metal tube at 5000 meters altitude that can break apart any second.