In the aftermath of the Boer War, the British military needed to address critical issues of practical marksmanship with its troops. The Long Lee rifles it had deployed to South Africa suffered significant problems in making real-world hits on the battlefield. In addition to investing in better musketry training, the military chose to radically change its rifles.

In place of the Lee rifles and carbines, a single short rifle pattern would be issued for all branches of service (cavalry, artillery, and infantry). A stripper clip loading system was introduced to speed reloading and a full-length handguard for improving bayonet handling and reducing sight mirage. A windage adjustable rear sight was mandated, and a stout full protective hood added around the front sight. A new nosecap design was implemented to put the weight of the bayonet onto the stock, and not on the barrel where it would impact the rifle’s zero.

Two different patterns of rear sight were considered. The A pattern design was a tangent type sight like a Mauser, pinned at the front. The B pattern used a ladder sight, pinned at the rear. Five hundred of each were made, and put through a rigorous set of remarkably practical field trials. The testing involved not just static shooting for accuracy, but also shooting against timed disappearing targets, camouflaged targets, and snap shooting. The trial winner was the A pattern design, and it went into mass production in 1904 as the Short, Magazine, Lee Enfield Mark I - the first SMLE.

As adopted a few minor changes were made from the trials rifles, most notably a change from a full front sight hood to a pair of stout protective Ewings, to allow more light onto the sight. In addition, the design was almost immediately updated to a MkI* pattern, with a stronger rear sling swivel, rounded corners on the receiver, and a storage trap added to the buttplate.