This model is of the first prototype of the infamous Ju 87 'Stuka'. Despite being powered by a foreign engine (a Rolls Royce Kestrel) and crashing with the death of the test pilot this design won the contract for Junkers against several competitors.
This model is a 'best guess' as the diagrams of this aircraft I have found are both incomplete and inconsistent.
The spherical recess underneath the model is for mounting it on a 4.5mm BB (.177").
Revised (21-Feb-2014) Reduced size of BB as model is light enough for a 4.5mm BB to be sufficient.