I had a print source say that FM 31-5 was almost a word for word copy of FTP 167. A footnote of the source below stated that FM 31-5 added sections dealing with disembarkation of cavalry and pack animals as well as withdrawal and re-embarkation of the landing force though. I have not researched, however, how the Army’s amphibious doctrine evolved throughout the war so I don’t mean to claim that it didn’t evolve, however based on evidence below and other sources I have, it’s clear that it at least did originate from Navy/Marine Corps Doctrine or at the very least that’s how the initial Army divisions were trained.
Web source evidence follows
“The positive results of his [Holland Smith] personal drive and outspoken advocacy for perfecting the conduct of amphibious operations led to his appointment as the commander of the First Joint Training Force in June 1941. This command, located at the new Marine Corps Base at New River, North Carolina, included the 1st Marine Division and the Army’s 1st Division. A similar organization was created later that year on the West Coast under Major General Clayton Vogel with the 2nd Marine Division and the Army’s 3rd Division. These units were directed to “plan, conduct, coordinate, and supervise all amphibious training in a series of exercises.” […]The newness of the regiments of the Army’s 1st Division required preliminary training prior to the exercise. During this preparation the Army requested copies of FTP 167 from the Navy and quickly adopted it with only slight modifications as Field Manual 31-5, Landing Operations on Hostile Shores on 2 June 1941 (page 77).”
“[Holland Smith boasted that] the first three U.S. Infantry divisions ever to become amphibious units, the 1st, 3rd and 9th, were trained by the Marine Corps; these were likewise the total of assault infantry divisions which executed our North African landings. Furthermore, in addition to these crucial three divisions, Marines trained the 7th, 77th, 81st, and 96th Infantry Divisions (page 80).”
Taken from “The Development of Amphibious Doctrine, a thesis presented to the Faculty of the U.S. Army Command and General Staf College in partial fulfillment of the requirements for the degree, Master of Military Art and Science, Military History.” by David C. Emmel, Major, USMC. 6 November 2010. www.dtic.mil/get-tr-doc/pdf?AD=ADA524286 (the link will download the PDF)
Furthermore, here is FM 31-5: http://www.ibiblio.org/hyperwar/USA/ref/FM/FM31-5/index.html
FTP 167 (which was adopted from the Marine Corps’ Tentative Landing Operations Manual of 1934/35): http://www.ibiblio.org/hyperwar/USN/ref/Amphibious/index.html