During world war 2 I was under the impression infrastructure and war related target were some of the main targets of air raids on both sides. What I don't understand is it takes months of not years to build bridges and factories, which I would assume could be destroyed in seconds with a handful of bombs. Wouldn't a single bombing run of a city be a huge devastation to a country's ability to produce weapons for the front? How affective were these bombing missions? Sorry if this is too vague or broad.
While bridges and factories do take a lot of effort and time to build, it is worth noting that bomb damage is very rarely total; and that many of these facilities have redundancies.
Take for instance the Ploesti raid. Despite a very large bombing raid against an oil refinery (which one would naturally think would burn up rather quickly), the raid actually destroyed only a small number of facilities that the Allies estimated to have caused a 40% drop in production.
In reality it was less, with the Axis reporting within a few weeks that there was no real curtailment in oil production:
http://www.fischer-tropsch.org/Tom%20Reels/Linked/B1870/B1870-0517-0541%20Item%208C.pdf
And the reason for this was twofold: First, the facilities damaged were mostly repaired within a few weeks. Second, there were back up and idle facilities, which then picked up the production from the facilities that needed long-term repair.
According to Ian Kershaw in "The End" (about the end of the war in Europe) the bombing campaign against Germany did very little to reduce Germany's ability to produce war goods. In fact, they increased their production as the bombing intensified. Some of this was to do with Armaments Minister Albert Speer being able to reorganise things to keep production going (including having to do with having fewer workers as Goebbels kept conscripting them into the Volkssturm). Another was the fact that factories weren't that easy to destroy. Sure, you could blow up a wall, take out a roof, etc, but the actual machines in the factory were pretty resilient. They weren't sensitive robotic equipment you see in today's factories, they were huge blocks of metal.
Going back to production, it was only in the final few months of the war that Germany's production crashed and that was due to loss of territory.
One of the fundamental problems is getting that handful of bombs onto the target; strategic bombing in World War II was highly inaccurate. Strong defences forced the RAF and Luftwaffe to bomb by night, making even locating a target city a challenge; in December 1942, the RAF calculated that one third of bombs landed within three miles of the aiming point in good weather; the average was 15% (and that was an improvement on 1941). The USAAF, bombing by day, found four-fifths of bombs landed within a mile of the aiming point in good weather, but between September and December 1944 only 14% of bombing was done with good to fair visibility, 10% with poor visibility, 76% was carried out non-visually. From every 100 bombs dropped on an oil plant, 87 missed entirely, only 2 hit the buildings and equipment.
(Figures from Richard Overy, The Bombing War: Europe 1939 - 1945, referencing The National Archives and The United States Strategic Bombing Survey.)