How to draw this shape progressively?

hi all,

please take a look at this picture :

i want to have this B appear progressively, following the shape lines (starting from the top edge of the most vertical trait), like a ribbon that progressively becomes the B, so i figured out that i could do it with masking and motion tweening but i can’t seem to make it work.

any idea on howto organize my timeline ?

thanks a lot,