Don...according to a quick google search:
http://history.amedd.army.mil/booksd.../appendixg.htm
About an accident involving cluster bombs being loaded into a B-17:
DESCRIPTION AND PERFORMANCE OF U.S. 20-POUND FRAGMENTATION BOMB
Construction
The M41 fragmentation bomb (fig. 4) has a charge-weight ratio of approximately 15 percent. Details of its construction are furnished by Prof. Marston Morse in his statement communicated to the Wound Ballistic Conference on 27 April 1944. The overall length of the bomb is 22.2 inches and its diameter about 4 inches. A long rod of square wire 0.44 x 0.44 inch is tightly wrapped about a light cylindrical casing 0.11 inch thick to form the main body of the bomb. The cylinder is filled with TNT or other explosives. The ends are sealed with steel plugs. The nose plug contains a cavity for an instantaneous fuse, and the tail plug has a threaded hole to take the tail fins.
FIGURE 4.-U.S. 20-pound (M41) fragmentation bomb.
The bomb is normally clustered into the M1A1 cluster of six bombs (fig. 5). The following loads, prescribed in USSTAF Ordnance Memorandum No. 3-54, 16 March 1944, were carried by aircraft in use by the U.S. Air Forces:
Aircraft/Load (*)
B-17: 38-42
B-24: 52
B-26: 30
B-25: 30
* In clusters of 6 bombs.
So it appears B-17s carried about seven x M1A1 clusters containing six bombs each for a total of 42 x M41s.
EDIT: This may seem small compared to the huge clusters you're showing for underwing P-47 applications; but remember that on the P-47; it's basically freefall, whereas in the B-17, you're constrained by the bomb bay walls; so for safety, the bombs have to kind of approximate the dimensional/c.g./c.o.m. of more normal bombs.