It is highly unlikely to be able to reduce you to a "B" cup from a DD without potentially compromising shape or viability of the tissue flaps or pedicle.
Based on the fact that your bra band size is 40, I am assuming your breast volume has something to do with your overall body size. If you want to be a B cup, you will likely need to change your entire body so your band size will decrease as well. Surgery cannot get you as small as you are considering. If your weight is an issue, consider weight loss first. Best wishes.