Would Robert E. Howard be considered historical fiction?
Certainly some of it might be. He wrote many Westerns, though most of those were set in his own period or shortly before; a lot of these can be found in the Bison volumes
The End of the Trail and
The Riot at Bucksnort and the Wildside Press
The Compete Action Stories. He also wrote a fair number of stories set during the Crusades, as well as other periods; a representative sampling can be found in the Bison
The Lord of Samarcand and the Wildside Press
Gates of Empire and
Treasures of Tartary. He didn't necessarily research his tales all that well, going on the histories he'd read over the years and a vivid imagination and (as de Camp once put it) "the underlying tragedy of life" of such periods. (A large number of his Westerns, however, were also humorous, more in the nature of tall tales, such as those concerning Breck Elkins or Jeopardy Grimes. His straight Westerns, on the other hand, tended to be quite stark and grim, and would make some very good films were a modern filmmaker to remain relatively faithful to the original material.)
On a tangential note, he also wrote a lot of sport stories -- especially boxing, which was a passion of his -- which were almost entirely set in his own period but which have now become closer to historical fiction in the sense of being a (highly fictionalized) peek at a past time....