I think everyone knows the image of a bunch of fresh-faced recruits getting screamed at by a surly drill sergeant and being made to go through grueling training.
However, when did this practice begin? I'm especially curious about how Medieval and Renaissance militaries trained their soldiers.
The idea that a large unit should have a person specifically tasked with drills is quite old. However, such a modern professional military organization can be traced, among other roots, to the Spanish tercio of 16th century Europe. The impact of the Spanish tercio organization is obvious, among other things it gave us the words infantry, colonel, etc.
This was a time when the combined-arms infantry rose to prominence, the significance of which is that soldiers were recruited from the common citizenry but were expected to eventually practice high skill levels. Unlike the previous French-dominated model of central and western Europe where a highly-skilled heavy cavalry came from the nobility class, accompanied by untrained, unskilled levy infantry.
Spanish interests in Italy and in the Low Countries meant they had significant bodies of soldiers, periodically reinforced by new recruits from Spain or from more local sources.
In particular, over time the tradition had it that new recruits were first sent to Italy for several years, before they are then sent to the front lines either in the Low Countries or Germany (or elsewhere). Thus, Italy became the training ground. Of course, they developed a rather detailed and formal procedure for training and education of soldiers, both through writing and oral traditions.
In particular, the tercio's second in command and the regiment's third is the Sergeant Major (Sargento Mayor in Spanish). His job are to determine the tercio's marching and fighting orders (therefore, one of his most important skills is the memorization of the square-root table!), and also to design and lead training. He had assistants who today would be called drill sergeants (then called ayudantes de sargento mayor), covering different soldiery types: pikes, arquebus, musket.
Source: Fernando Gonzales de Leon, The Road to Rocroi.
In short, the official “Drill Sergeant/Drill Instructor” position is an advent of the 20th century. My sources are U.S centric, for which I apologize, and I await some of our European historians to contribute as well.
The roots of the Drill Instructor appear to be traced to the United States Marine Corps in 1911. Here, Major General William Biddle, 11th Commandant of the Marines, had required an assessment of recruit training and found it to be inadequate, with different commands producing different caliber Marines. Commandant Biddle required the Marines to centralize their recruit training in order to produce consistently high quality recruits, and formed Recruit Training Depots in Norfolk, Va.; Philadelphia; Mare Island, Calif. and Puget Sound, Wash. It appears that by 1923, the Marines’ recruit training had been boiled down to two Recruit Depots, one at MCRD Parris Island (North Carolina), and the other at MCRD San Diego, which is the situation as it stands for today: “By 1915, East Coast recruit training was centralized at MCRD Parris Island, S.C. In 1923, MCRD San Diego assumed the duties as sole recruit depot for the West Coast.” Continuing on, we begin to see the formation of Drill instructor schools for the Marines; although the needs of World War II were very heavily taxing on these as the need for senior and experienced war fighters to return home and train new recruits outstripped the Marines’ Drill Instructor school’s abilities. The idea behind the Drill Instructor school was that it created a training regimen for the trainers: they only became qualified D.I.s after passing the school; this ensured the production of similarly trained Marine instructors who could then train recruits to the high caliber that the USMC demanded.
The United States Army underwent a similar remodeling in 1962 after a survey requested by then Secretary of the Army Elvis J. Stahr Jr. revealed that the US Army’s training abilities were far below the standards set by the other four services (USMC, USN, USAF, USCG). One of the root causes found for low quality recruits and low morale was the poor quality of the NCOs who were being sent to the recruit depots to train the recruits: These NCOs were often harassing, demeaning, and concentrated more on humiliation and demoralization then training, unit building and esprits de corps. This low quality of NCOs was seen to be caused by units farming out their worst NCOs to clean that home unit out. The Army realized that it needed to form an elite school dedicated to professional recruit instructors, similar to that of the Marines’, and did so with the formation of their Drill Sergeant schools: “The Fort Leonard Wood Drill Sergeant School began training NCOs for drill sergeant duties in September of 1964.”
On a funny side note: Marines hate being called “Drill Sergeant”, it is seen as an insult; all Marines in the Drill position have the title of “Drill Instructor”. “Drill Sergeant” belongs to the US Army.
So what came before? Well, it appears that often the recruits would be placed in their unit first, and trained by the existing NCOs or senior men there.
For Western militaries, training often seemed to be under the purview of senior, experienced men who had already seen line combat, and then would pass on that unit's experiences and culture to the new recruits.
Sources:
http://www.army.mil/drillsergeant/history.html
Ricks, Thomas E. “Making the Corps”; (Read this book if you want to know about U.S.M.C . D.I.s