Why did the B-29 have so much leftward torque at take-off? In this book, The Making of the Atomic Bomb by Richard Rhodes (p. 705), I read:


  [Paul Tibbets] eased the brakes [of Enola Gay] at 0245, the four fuel-injected Wright Cyclone engines pounding. “The B-29 has lots of torque in take-off,” he notes. “It wants to swerve off the runway to the left. The average mass-production pilot offsets torque by braking his right wheels. It's a rough ride, you lose ten miles an hour and you delay the take-off.” Nothing so rude for Tibbets. “Pilots of the 509th Group were taught to cancel torque by leading in with the left engines, advancing throttles ahead of the right engines. At eighty miles an hour, you get full rudder control, advance the right-hand engine to full power and, in a moment, you're airborne.”


I am not especially knowledgeable in aviation matters, but I believe that the torque is due to the B-29's four propellers all rotating in the same direction. Is that right? If so:


Why did this problem only arise during take-off?
Why did all propellers rotate the same way? Is this true for all propeller-driven aircraft with more than one engine? Why?
If the 509th Group's technique was so good (and apparently not especially complex), why did only they use it?
It didn't arise ONLY during takeoff, but it is most noticeable then for several reasons


The engines are at maximum power, therefore producing maximum torque
You've got a very good reason to need to be travelling in an exactly straight line (the runway)
You're travelling at fairly slow speed, so your vertical stabilizer is less effective - at higher speeds, the tail counteracts the torque more effectively (for this B-29 this occurs at 80mph, as mentioned). As soon as you get above 80mph, you can just input a little right rudder to counteract it.... B-29's only travelled below 80mph on takeoff and landing, so it wasn't a problem during flight, and during landing the throttles were at idle or very low power.

It isn't true for all aircraft with more than one engine now (although it is true for some), but it does still happen and was much common back in WWII - they didn't want the extra supply line complexity of two different sets of otherwise identical engines, when it could be solved with a bootfull of right foot. Note also that the B-29 had 4 engines, most modern multi-props only have two, and they're closer to the fuselage, which reduces the effect
I think you're misinterpreting the source: he's not saying this technique was unique to the 509th, just that they were taught the technique. The very fact he says they were taught it, rather than "came up with" it, suggests it was known elsewhere. The 509th are only referenced here because that's the group Tibbets was part of, it's unrelated.