I initially assumed the challenge was to fit the entire banana on the bread in a single layer; I am disappointed that problem was not solved, but it seems like it should be a relatively trivial extension of the package provided (produce a packing for [N, M] slices of banana, pick the number of slices which covers the bread without any left over pieces).
(Of course, as someone who does not eat banana sandwiches, I have no idea of such an end is even desirable.)
If you want to make a PB&B that uses the entire banana, simply peel the banana, coat the flesh in PB, tear up your bread slices and adhere them to the banana.
This method is not guaranteed to use all the bread.
Smear peanut butter on the bread, NOW tear up the bread, and apply the peanut-butter-smeared shreds to the (peeled) banana.
Still the same primary advantage as pliny's method, same primary drawback (may not exhaust the bread), but we've solved the structure problem that you raise.
I feel the trend-line here is looking good; with a few more iterations, we may reach the true ideal PB&B sandwich.
(Of course, as someone who does not eat banana sandwiches, I have no idea of such an end is even desirable.)