Ready for some surprises? According to the Illustrated Encyclopedia of Military Aircraft, Plate 114, Bomber Production in the Second World War, the top 10 are:
B-24 18,188
Ju.88 14,980
Vickers Wellington 11,461
B-25 11,000
B-17 8,685
Heinkel He 111 7,450
A-20 7,385
Lancaster 7,366
Curtis Helldiver 7,002
Tupulov SB-2 6,600
However, the plate is obviously incomplete, omitting such bombers as the Il-2, Pe-2, Tu-2, and many others.
Other sources say the Il-2 was the most produced aircraft in WWII with more than 35,000, although it isn't clear how many of those were produced after WWII and how many were IL-10s.
Whatever, consensus seems to be Il-2 was the most produced bomber in WWII by a considerable margin.
In the above plate, biggest surprise to me is the Helldiver with 7,002. Some sources denigrate the Helldiver, others claim it was important and effective.