Yes, it is necessary to apply for a tourist visa in one of the U.S. Embassies in India at least two weeks before your trip. Also, as most flight connections between India and the United States take place in countries in Europe and West Asia. Passengers opting for non-direct flights for those stops would need a separate visa, a transit visa, as it’s the case with those flying through Frankfurt, London and through Dubai if it is a transit of 48 hours and 96 hours.