XXXXI Reserve Corps was formed in December 1914. It was part of the second wave of new Corps formed in the early stages of World War I consisting of XXXVIII - XXXXI Reserve Corps of 75th - 82nd Reserve Divisions (plus 8th Bavarian Reserve Division). The personnel was predominantly made up of kriegsfreiwillige (wartime volunteers) who did not wait to be called up. It was still in existence at the end of the war.
On formation in December 1914, XXXXI Reserve Corps consisted of two divisions. but was weaker than an Active Corps
the divisions were organised as triangular rather than square divisions with three infantry regiments rather than four, but had a brigade of two field artillery regiments
Reserve Infantry Regiments consisted of three battalions but lacked a machine gun company
Reserve Cavalry Detachments were much smaller than the Reserve Cavalry Regiments formed on mobilisation
Reserve Field Artillery Regiments consisted of two abteilungen (1 gun and 1 howitzer) of three batteries each, but each battery had just 4 guns (rather than 6 of the Active and the Reserve Regiments formed on mobilisation)
In summary, XXXXI Reserve Corps mobilised with 18 infantry battalions, 2 cavalry detachments, 24 field artillery batteries (96 guns), 2 cyclist companies and 2 pioneer companies.
Cron, Hermann (2002). Imperial German Army 1914-18: Organisation, Structure, Orders-of-Battle [first published: 1937]. Helion & Co. ISBN1-874622-70-1.
Ellis, John; Cox, Michael (1993). The World War I Databook. Aurum Press Ltd. ISBN1-85410-766-6.
Busche, Hartwig (1998). Formationsgeschichte der Deutschen Infanterie im Ersten Weltkrieg (1914 bis 1918) (in German). Institut für Preußische Historiographie.
Histories of Two Hundred and Fifty-One Divisions of the German Army which Participated in the War (1914-1918), compiled from records of Intelligence section of the General Staff, American Expeditionary Forces, at General Headquarters, Chaumont, France 1919. The London Stamp Exchange Ltd (1989). 1920. ISBN0-948130-87-3.
The German Forces in the Field; 7th Revision, 11th November 1918; Compiled by the General Staff, War Office. Imperial War Museum, London and The Battery Press, Inc (1995). 1918. ISBN1-870423-95-X.