The B-29 had an increased range and payload, and when the bombing campaigns were ramped up over Nazi Germany and losses of B-17s and B-24s were mounting, why didn't planners attempt to try the B-29 in European skies? You'd think if you were going to lose aircraft, then get a bigger payload over your target to dish out more punishment as you are losing aircraft either way.
They didn't need the range in Europe as much as they did the Pacific. The B-29s needed bigger facilities and improved runways than the B-17s , B-24s and Lancs were using in England.
I seem to recall reading that most of the existing airfields wouldn't support them without considerable work. Then there were the distance in the Pacfic that only the B-29 could handle. Production was rather slow at first as well. From wiki at: http://en.wikipedia.org/wiki/Boeing_B-29_Superfortress#World_War_II Over optimistic assesements of what would happen in Europe may also have played a part note (from the source above): It may well have been assumed that by the time they got a significant mass of B-29's in Europe Germany would have surrendered. Especially since B-17s and B-24s among others seemed to be doing the job in Europe.
One B-29 did come to the UK late in the war and toured the 8th AF bases, causing much interest. As stated above, though, it was too late in the game to create the necessary logistical systems. The much greater range of the B-29 was more useful for the Pacific.
The 20th AF conducted its first raid on Tokyo from the Mariannas in mid-November 1944. At the end of the month, the 20th had 262 B-29s. At the same time, the 8th AF and 15th AF had 4,894 B-17s and B-24s. Those few B-29s were the only aircraft that could conduct those raids from the Mariannas. They wouldn't have made much of a difference in Europe.
While the Luftwaffe never ceased to be a threat, by the time the B-29s were ready for operations, we were getting it under control in Europe, due to the advent of long-range escort fighters and the consequent rapid degradation of the German fighter force. And of course you never get something for nothing. B-29s in Europe would mean fewer for the Pacific where it was considered they could make a real difference. The logistics to support them in Europe would be at the expense of something else - existing bombers, tactical air, ground troops, something.