"The boys are eating a good bread."
Ugh. It's usually "du" because it's an unknown quantity. But, I guess the descriptor is the reason it's "du".
"a good bread" is something specific, where as "bread" is not. In French, it is the same: "un bon pain" (as opposed to baguette or other speciality) vs "du bon pain" (bread in general)
Is it often that someone will say "a bread", generally it is a loaf of bread or a slice of bread. Bread itself is usually not a singular in english, is that different for french?
"Un pain" is either a loaf of bread or a thick baguette (450g or 500g vs 250g). "Un petit pain" is a bun, "un pain de campagne" is made with a specific flour (and with flour spread on the crust), and there are still a variety of other specialities with the same construction.
If unspecific, it is "du pain" (bread or some bread).