Calling something an army bag may refer to a number of different containers meant to carry equipment, tools, survival and tactical gear, as well as medical supplies and MREs.
There are multiple types of army bags. An army backpack is called a rucksack. Rucksack is a German word for backpack and was established in the US during the civil war.
Rucksack is same as the backpack and generally refers to larger backpack models used by the army or other forces. It is meant to carry multi-purpose gear.