It depends on the thickness and the moisture content in the wood. As Pearl said, heat for dry, steam for wet wood. Certain woods though respond better with steam than with heat depending on the thickness and the amount of bend you trying to make. For most bow woods, if the wood is dry, I would try dry heat (heat gun) first. Let the wood cool down for some time on a form or caul before stressing it. If not, more heat might be needed. It is not unusual for you to need to bend a little more than what your final bend is intended to be at. Steam will allow you to form nearly any thickness and radius desired with nearly any species but it is a bit more complicated to do.