I messed up °F/°C and it was "only" 250°F. I don't have the book "Space Medicine" available, but I found a different reference† explaining that it was Dr. Charles Blagden and other researchers in 1774 who went into a room heated above 250°F and cooked steak and eggs in there, proving the importance of perspiration in maintaining body temperature. It has to be dry heat; if there is humidity, the tolerable temperature is much lower. The original paper ‡ is also online; in order to read it, note that "ſ" is a "long s", not a "f".

† http://news.google.com/newspapers?nid=1499&dat=19560213&id=4...

‡ http://rstl.royalsocietypublishing.org/content/65/484.full.p...