I don't know how big that bag is - is it a hard sided bag? I know that my wheeled carry-on size bag (22" I think but going by memory) would never fit under the seat - it has to go in the overhead.
Why not just plan on putting it in the overhead?
Under the seat space is quite tight, and most things have to be smooshed a bit to get them underneath, unless there about the size of a laptop computer bag.