Bombs have a blast radius, and if withing the blast radius stuff will go boom (if the bomb was of the required size or more). Thus, a drop of 1000lb' on a town center will kill a lot of buildings, and if you land your bombs between two city blocks or right on an intersection, you may kill (with some luck) 2-4 city tiles.
It's just more difficult as the City tiles are much bigger targets and require 844lbs of destructive power (equals about 3x250lbs bombs).
Bombs larger than 1k are usually a total waste on the City, even 1k bombs are only efficient if you really manage to hit an intersection on point. More efficient are 500lbs bombs and flying down one aisle dropping right on the street between 2 blocks, or taking 250lbs bombs and dropping right on each block (the 250lbs are not very suited for bombing the factories though, so you should exclusively concentrate on the City itself then)
(All this assumes flying a bomber formation, if you fly a single 29 for example you'd have to adjuts for it)
The strats (City & factories) determine the downtime of items on bases. Basic downtimes are 30 minutes for barracks, radar, auto guns, ord bunkers and town buildings, 45 minutes for HQ - hitting the corresponding factories (city for town buildings and HQ) beforehand will increase that downtime by up to 120 minutes.
Destruction of factories has no impact on any items already down, or any hangars or manned ack at all.