Info on a Pilot/Col. Langford

Hi,
My question is this: Was it normal to fly at 12,050 ft when flying over a bomb target?  I was under the impression that B-17s typically flew at 20-23 thousand ft?  That they didn't fly low because it made them a target to flak hits?
 
My Uncle Fred Beach was killed while flying a mission over Belgrade, Yugo, May 7, 1944.  COl. Langford choose most of his crew to fly this mission.  I have now learned from a ball gunner that witnessed the plane being hit by flak and going down that Langford was flying way to low over the bomb target.  I believe at about 12,050 ft.  In the MACR Langford states he was at 20,000 ft when hit. 
 
I found in another forum conversation that Langford was a part of another low formation where 2 B-17s were shot down.  This was months earlier. 
 
I don't want to point fingers or anything, just try to understand why and how this happened.  If anyone can help me understand more about this and more about Langford, I would really appreciate the education.
 
You all have been so helpful to me and I want to thank you upfront!
Cathi