With regard to the first published use of "life's a bitch," the case for Langston Hughes, The Big Sea (1940) seems pretty strong. The next-oldest instance of the expression in that form that I've been able to find is from Kenneth Patchen, Sleepers Awake (1946):
If I say it over and over, then maybe I'll go to sleep . . . My name is Tranquil Flume. I am twenty-two years old. Oh, my name is Flume, Tranquil Flume and I am twenty-two years of age. Of young. Of life. Of beauty. What shall I wear tomorrow? Dark blue dress I bought in Boston. O the dark blue dress. Tranquil's dark blue dress. The very one she bought in Boston which used to be a nice city to live in O why can't I cry!
(She crosses to the window. Sometimes life is a bitch, huh? How about it? don't you think so?)
However, expressions equating life with a female dog go back much farther than the 1940s. The oldest of these seem to be concentrated around versions of the phrase "bitch of a life" and very possibly to have originated in English translations of French texts. Here are some English translations, organized in chronological order, that use the expression "bitch of a life." From an 1845 translation of Eugène Sue, Latréaumont [translated as De Rohan; Or, the Court Conspirator] (1837):
And then once on horseback, and assembled under the eyes of that Saint Aignan—hell seize him—they had no other voice left, the cursed apple-munchers, than to bray out, 'Long live the king;' God's-death; and shout that no one should dare to shake their fidelity. By hell, what have they got to lose?—their bitch of a life?—and that to be sure is very much to be regretted.
From a 1927 translation of Anatole France, The Shirt (1909), in Stuart Sherman, The Main Stream (1927) [combined snippets]:
"... As for myself, I love life, the life of this earth, life such as it is , this bitch of a life. I love it brutal, vile and coarse; I love it dirty, indecent, tainted; I love it dull, idiotic, cruel; I love it in its obscurity, its ignominy, its infamy, with its stains and uglinesses, and stenches, its corruptions and infections. Feeling that it escapes and flees one, I shake like a coward and become mad with despair."
From Haakon Chevalier, The Ironic Temper: Anatole France and His Time (1932) [combined snippets]:
Also, La Vie en Fleur [1922 (according to Wikipedia)], p. 319: (M. Dubois' last speech) "I have not tasted in more than three-fourths of a century, a single day of happiness. Hear that, my friend: although fate has spared me the great evils of which it is prodigal to so many mortals, though I have experienced neither cruel illness, nor griefs which nature condemns, I would not begin again a single day of my life. And yet, I tell you, I doubt if I do not expect, against all reason, some good, some pleasure out of life, beyond whose ordinary span I have already passed. And I am bound to recognize, if not by personal experience, at least by reasoning, that this bitch of a life (the word is Madame de Sévigné's) has sometimes an element of good, even though I have not observed it."
Theodor Reik, The Need to be Loved (1963) seconds France's attribution of the phrase "this bitch of a life" to Madame de Sévigné, although his translation differs a bit:
Old age wishes that all should be as it is and wishes nothing better. It is conservative because the change it foresees can only be for the worse: disease and death. In spite of its faults and shortcomings it loves life, or at least it wants to stay alive as long as possible, to remain loyal to "that bitch Life" (the expression "cette chienne de vie" is from Madame de Sévigné) because it is the best of its kind we know.
The [Yale] Dictionary of Modern Proverbs (2012) seems skeptical about the assertion that Sévigné was the source of the expression, implying that no one has pointed to a place in her works where the expression occurs:
Life is a bitch. ... The phrase chienne de vie ("bitch of life") was attributed (without authentication) to Mme. de Sévigné by Anatole France, La vie en fleur (Paris: Calmann-Levy, 1926) 319.
But "chienne de vie" (preceded by "quelle" rather than "cette") does occur in Lettre LIII (dated March 27, 1735) of "Lettres de Madame La Marquise de Simiane," reprinted in Marie de Rabutin-Chantal marquise de Sévigné, Lettres nouvelles ou nouvellement recouvrées de la Marquise de Sévigné, et de la Marquise de Simiane, Sa Petite-Fille (1774):
Diantre! comme vous allez vous goberger à ce B...! quelle chienne de vie! n'y oubliez pas tout-à-fait les pauvres solitaires d'Aix. Embrassez pour moi ce pauvre D., je vous en prie, je vous le rendrai ici ; mais peut-être ne serez-vous pas touché de cette restitution ; vous aimeriez mieux celle des S... Je vous la souhaite, Monsieur.
[Google-translated roughly as "Damn! how you are going to swallow this B...! what a bitch of life! do not quite forget the poor solitaires of Aix. Kiss poor D. for me, I beg you, I'll give him back to you here; but perhaps you will not be touched by this restitution; you would prefer that of the S... I wish it to you, sir."]
Madame de Simiane was Madame de Sévigné's daughter.
Also, from a 1935 translation of Roger Vercel, Captain Conan (1934) [combined snippets]:
Or it might have been better if you'd got me sent to a penal battalion, that time you were handing out tickets, you know, along with Beuillard and Grenais, those old pals of mine. ... Well, life's like that, as you said: this poor old bitch of a life. ..."
From a 1936 translation (by Walter Owen) of José Hernández, The Gaucho Martin Fierro (1872/1879):
And the filthy state we soon were in, / Was horrible to see; / For pity's sake one's heart might break, /By Christ! it might give you a belly-ache, / I never in all my bitch of a life / Saw greater misery.
From a 1937 translation (by Allan MacDougall) of Lilika Nakos, "The House on Fire," in Life and Letters To-Day, volume 16, number 7 (Spring 1937) [combined snippets]:
"A bitch's life! A bitch of a life!" he muttered as he went downstairs. On the outside steps there was a pot of begonias. With a swift kick he sent it crashing into the neighbouring courtyard. Once in the street he strode quickly off.
This is the instance noted by Pete in his answer.
A somewhat different expression appears in a 1936 translation of Roger Vercel (again), Remorques [translated as Salvage] (1935) [combined snippets]:
From that day onwards, Kerlo had given up drinking. Instead he had taken to reading—and that was much worse! All he read was books by authors who were tired of life. Then he would tell the men that, when one had lived long enough, as he had, to know how like a bitch life could treat the best men in the world, the best thing to do was to jump overboard.
The earliest similar home-grown US English expression seems to involve the phrase "son-of-a bitch of a life." From "Destroyer Life," (circa 1917–1919) in Alan Lomax, American Ballads and Folk Songs (1934):
Oh, it's roll and pitch / And creak and groan, you son of bitch. / Oh, boy, it'ds a hell of a life on a destroyer. / Oh, Holy Mike, you ought to see / How it feels to roll through each degree. / The God-damned ships were never meant for sea. / You carry guns, torpedoes, / And ash-cans in a bunch, / But the only time you're sure to fire / Is when you shoot your lunch. / Your food it is the navy bean, / You hunt the slimy submarine. / It's a son-of-a-bitch of a life on a destroyer.
From Fred Rothermell, To Raise These Halt (1936):
Laundry-driver Mahoney: ear ache and constipated and dizzy and spots before his eyes. Camphor drops and a dose Cascara. Mrs. Winrop and her chest pains. A note to the Stillman Laboratories. Front and rear exposures: pictures to him. The phone bell. "God damn son-of-a-bitch-of-a-life. The bastards!" He clutched his ears, tearing till the pain sobered him.
But then, from MacKinlay Kantor, The Noise of Their Wings (1938) [combined snippets]:
"I suppose it did. He never showed that side of himself to me. Maybe," she said to Holm Hansen, "he showed it to you. The other side—the side of him which would care, and would feel—would feel softness and grief. You might think that I would have had an easy life—an idyllic existence. Well, I haven't. I've had a bitch of a life."
Conclusions
The print record for "life is a bitch" may go farther back than 1940, but I couldn't find any thing older than the instance from Langston Hughes, cited in the OP's question.
On the other hand, English translations of French works using the expression "bitch of a life" go back at least to 1845, in a translation of Eugène Sue's Latréaumont. And it seems to me to be at least possible that an instance of chienne de vie from 1735—sometimes attributed to Madame de Sévigné, but actually written by her daughter, Madame de Simiane—may have contributed to popularization of the turn of phrase in France long before it crossed over into English in the form "bitch of a life."
In any event, the concentration of early instances of "bitch of a life" in translations from French sources (along with one instance from an Argentinian source and one from a Greek source) strongly suggest that the expression arose as an import from France, rather than as a native-born Anglicism.
