On Flikr there are several photos of the North American NA-16-2K from San Diego Air Space Museum showing the aircraft in California before it was delivered to CAC in Australia:
It's hard to tell for sure, but those are probably practice bombs, some as small a 25 lbs, but they had the same ballistics as their big brothers. They had a small charge that made a big smoke puff so their strikes could be observed.
In the Aviation section post 90 in the thread, bomber offensive vs Germany, there is a chart of US WW2 bombs. It list some 100 lb bombs, including a practice bomb.
Those bombs do look too big for 25 lbs, could be 100 lbs bombs of some sort.
A 1938 edition of "Jane's" has descriptions of several different NA-16 types including a "bomber" version. Bomb load is given as four 100lb or ten 25lb bombs. Also a single seat "fighter" version that can also carry four 100lb bombs.