Hey I actually work in a part of an air museum dedicated to "Flying the Hump". Most likely, it looks like he would have been flying a Curtiss C-46 Commando. This thing is huge. It was the largest twin engined airplane of the war AFAIK. It's wingspan was 108 feet and each engine was the famed Pratt & Whitney R-2800 which put out about 2000 horsepower each. This was an incredible engine used in several airplanes during the war including, but not limited to the Republic P-47, Grumman F6F, F7F, and F8F 'cat' fighters, P-61 black widow, and more. The airplane had a technical service ceiling of about 28k feet. A pilot I spoke to reported he was never able to reach any higher than 26,500 feet. This is all very impressive for the day, as the Douglas C-47 could carry half as much cargo as the C-46, and couldn't fly as high due to smaller engines and smaller wings. Like other commenters said, these men flying the hump were often flying cargo missions to China. Although not only to Chiang Kai-Shek. They also supplied various American Air Forces stationed in China. Air Forces such as the Flying Tigers (American Mercenaries hired by the Chinese to repel the Japanese before american involvement) and in 1943-4 the first B-29 bombing missions to Japan. Not to mention supplying various P-51 and B-24 squadrons to help with the island hopping and attacking the Japanese. Also, as /u/DuxBelisarius pointed out, these missions were very inefficient. These C-46's had to carry in thousands of gallons of fuel (which was often used up on the way back) but troops, aircraft parts, bombs, and other weapons had to be flown over the Himalayas. It was said that there was no "best route" to take over the Hump, as every day enemy positions and weather conditions changed and made missions very dangerous. Apparently you could almost follow the wreckages of C-46's to your destination. I head back in on the weekend, and there are charts, pictures and a map that I can show you if you're interested! Your grandfather was a brave man to fly those missions, and you should be proud of him.