On June 15, 1933 Howard wrote to Hugh G. Schonfield (letter #248) sending them a collection of weird stories hoping them to be published in a book:

As I promised, in answer to your letter of May 19th, I am sending, under separate cover, a representative collection of my weird stories. These include:

1. Wings in the Night
2. The Tower of the Elephant
3. Kings of the Night
4. The House of Arabu
5. The Valley of the Lost
6. The Scarlet Citadel
7. The Horror from the Mound
8. The Children of the Night

As I have no typed copies of “The Tower of the Elephant” or “Wings in the Night,” I am sending the printed pages of these stories in order to save time. If you wish, I will prepare manuscript copies of them.

“Wings in the Night,” “The Tower of the Elephant,” “Kings of the Night,” “The Scarlet Citadel,” “The Horror from the Mound,” and “The Children of the Night” were published in Weird Tales magazine. I own the British Empire rights. “The Valley of the Lost” was accepted by the Clayton Publishing Company, for their magazine Strange Tales, but that magazine was discontinued before the story was published, and it was returned to me. “The House of Arabu” has never been published.

In a letter dated January 9, 1934, managing director Hugh Schonfield of Denis Archer, Publisher, returned the stories Howard had sent them, attributing the difficulty in publishing the collection to “the prejudice that is very strong over here just now against collections of short stories.” However, he said, “any time you find yourself able to produce a full-length novel of about 70000-75000 words along the lines of the stories, my allied Company, Pawling & Ness Ltd . . . will be very willing to publish it.” 

To August Derleth (letter #276) circa January 1934 Howard writes:

An English firm, after keeping a collection of my short stories for months, finally sent them back, saying that there was a prejudice over there just now against such collections — of short stories, I mean — and suggested that I write a full length novel for them.2 But I’m not overly enthusiastic about it, for I’ve been disappointed so much. Of course, I’ll do my best.

In a letter (#285) to Denis Archer from May 20, 1934 we learn:

Dear Sir:
As you doubtless remember, in your letter of Jan. 9th., 1934, you suggested that I submit a full length novel, on the order of the weird short stories formerly submitted, to your allied company of Pawling & Ness Ltd.

Under separate cover I am sending you a 75,000 word novel, entitled, The Hour of the Dragon, written according to your suggestions. Hoping it will prove acceptable, I am, Cordially,

In another letter (#301) to August Derleth from mid-October 1934, we learn:

I haven’t yet gotten a copy of the Terror by Night, but intend to shortly. Just got a letter informing me that the English company which had promised to bring out my book had gone into the hands of the receiver. Just my luck. The yarn’s in the hands of the company which bought up the assets, but I haven’t heard from them.

The Terror by Night was an anthology in the Not at Night series, edited by Christine Campbell Thomson and published in the UK by Selwyn & Blount. This title included Howard’s “Rogues in the House.”

Howard sold the story to Farnsworth Wright (Weird Tales) and in a letter (#307b) to H.P. Lovecraft, ca. January 1935 he says:

P.S. Not long ago Wright accepted a 73,000 word serial novel, tentatively scheduled for some time in 1936. A Conan yarn, The Hour of the Dragon. Wright says it’s my best Conan story so far, and I agree with him. I hope you like the yarn.

In a letter (#330) to August Lenninger) on December 27, 1934, Howard mentions the story:

I write westerns, adventure, fantasy, sport, and occasionally detective. I have been a contributor to Weird Tales for eleven years, and a 70,000 word novel, The Hour of the Dragon is at present running in that magazine as a serial.

August Lenniger was the agent who handled the proposed anthology of weird tales that Price and Kirk Mashburn had proposed. 

From a letter (#342) to P. Schuyler Miller, March 10, 1936 we learn a bit of Conan's timeline:

Conan was about forty when he seized the crown of Aquilonia, and was about forty-four or forty-five at the time of The Hour of the Dragon. He had no male heir at that time, because he had never bothered to formally make some woman his queen, and the sons of concubines, of which he had a goodly number, were not recognized as heirs to the throne.

and in the same letter, he comments on Vincent Napoli's artwork:

Yes, Napoli’s done very well with Conan, though at times he seems to give him a sort of Latin cast of the countenance which isn’t according to type, as I conceive it. However, that isn’t enough to kick about.

To Weird Tales, in a letter (#343) circa April 1936, he has nothing but praise for Vincent's work:

Enthusiasm impels me to pause from burning spines off cactus for my drouth-bedeviled goats long enough to give three slightly dust-choked cheers for the April cover illustration. The color combination is vivid and attractive, the lady is luscious, and altogether I think it’s the best thing Mrs. Brundage has done since she illustrated my “Black Colossus.” And that’s no depreciation of the covers done between these master-pictures. I must also express my appreciation to Mr. Napoli, who has done a splendid job of illustrating my serial. I hope the readers have liked the yarn as well as I liked writing it.