bombing towns at fields does nothing except help to capture it.
bombing the strategic factories is a different matter
any damage to a strat slows the rebuild of that strat on fields ie:
bomb the radar factory, then kill the radar at an airfield, the time it takes for the radar at that airfield to rebuild is longer depending on how damaged the strat is.
bombing the large city effects the rebuild time of all the strats themselves.
nothing ever changes the downtime of the hangars, and nothing ever speeds up the rebuild of hangars likewise.