There are only 3 rack settings in my gas oven. I have baked on the top rack and my bread is raw. Which one should I use to cook at 350f for 35-40 min.
I have a gas oven as well, and have found that baking on the middle rack, using a pizza stone helps bread bake more evenly. Additionally, as User22259 suggested, adjusting the baking time until the internal temp of the bread reaches about 220-degrees will get you a perfect product every time.
Turn the temperature up. 350F is not hot enough for bread -- try something more like 400F (200C). Also, check that your oven calibration is accurate -- see this answer for ways to check it.