Stealth Bomber Promotion XP Limit on Cities?

[timtofly]

I have a Stealth Bomber that will not gain any more points when attacking cities, but does get points when attacking units. Is there a hard limit for either and does any one know what they are? I know that Barbs only allow up to level 2. Is there a bug or a hard limit on other actions?

I have reached level 14 and the count stopped at 1000 on the way to 1050.

Thanks.