I would imagine a pretty decent one if you want any kind of precision when you're 10km up in the air. You need to take your own ground speed into consideration, as well as the windspeed as the bomb is going to be in free fall for around 3/4 of a minute.
Getting your ground speed would have been a real pain since you would have to have calculated it from your indicated air speed and guessed at the air density and winds aloft. Or you can time it by using fixed reference points below you... except those were being obscured and are still remarkably easy to get wrong when you're 10KM away.
You just need one reference point to measure your groundspeed, and you don't have to know what it is, you can just pick any arbitrary feature below you. With a known altitude, measuring the angular velocity of a ground feature gives you your groundspeed. There were mechanical systems available by the 1920s or 30s that could do this automatically, perhaps most famously the Norden bombsight: http://en.wikipedia.org/wiki/Norden_bombsight
Still, if you want to hit a target like this, you have to be able to see it, and see enough context around it to be able to recognize it.
That's a fascinating article. It does mention that they weren't particularly useful though:
_Over Japan, bomber crews soon discovered strong winds at high altitudes, the so-called jet streams, but the Norden bombsight worked only for wind speeds with minimal wind shear. Additionally, the bombing altitude over Japan reached up to 9,100 metres (30,000 ft), but most of the testing had been done well below 6,100 metres (20,000 ft). This extra altitude compounded factors that could previously be ignored; the shape and even the paint of the bomb mantle greatly changed the aerodynamic properties of the weapon, and, at that time, nobody knew how to calculate the trajectory of bombs that reached supersonic speeds during their fall_
Note that this wasn't due to trouble with calculating groundspeed, it just turned out that figuring out where a bomb would hit requires way more than just altitude, groundspeed, and position. Still, the fact that it's complicated is quite relevant here, especially since the bomb was dropped with a parachute. Somewhat counterintuitively, bombing accuracy was a pretty big driver behind a lot of the nuclear arms race, and improving accuracy is why bombs increased in power rapidly up to the 60s, then gradually declined.
Getting your ground speed would have been a real pain since you would have to have calculated it from your indicated air speed and guessed at the air density and winds aloft. Or you can time it by using fixed reference points below you... except those were being obscured and are still remarkably easy to get wrong when you're 10KM away.