Question: Which one is appropiate?

> 
> I will submit my assignments by tonight
> 
> 
> I will be submitting my assignments by tonight.
> 
> 
> 

I want to express that before the end of the day, I will submit my assignments.

Answer: 
First off, with the preposition *by*, we would normally mention a specific time or point in time. I will submit my assignments by 10 tonight or by midnight tonight. Or you can say *by the end of the day*, because that also refers to a specific point in time. (*By tonight* does not really specify a point in time, unless there is a previously defined specific time that *tonight* refers to, such as a deadline that everyone knows about.)

As far as which construction to use, either one is fine. The first one has more of a promise meaning. The second one has a more matter of fact meaning. I explain the difference in detail [here](https://ell.stackexchange.com/a/48043/6951)
1 will submit

2 will be submitting

In my view , the formula 2 here is not used for stressing the progressive aspect but simply to give the idea "will submit" more weight, simply because the formula "will be submitting" is longer. Whether this emphasis of the submitting act is appropriate or not is another question. I have the feeling "will be submitting" sounds a bit like a cheap rhetoric effect.