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The Sten was created during a time of desperation. The British military had suffered great materiel losses during the evacuation at Dunkirk, and needed a submachine gun that could be quickly manufactured to help resupply the armed forces. At the time, the British had been purchasing large quantities of Thompson submachine guns from the United States. However, Thompsons were time-consuming and expensive to produce, being anywhere from $70-200 per unit. As a result, the guns could not be produced fast enough, and so the Royal Small Arms Factory Enfield was commissioned to design an alternative.<ref name = "sten modernfirearms">[https://modernfirearms.net/en/submachine-guns/great-britain-submachine-guns/sten-eng/ Modern Firearms entry on the STEN]</ref>
The design of the Sten gun is credited to Major Reginald V. Shepherd and Mr. Harold John Turpin. Major Shepherd was the inspector of Armaments in the Ministry of Supply Design Department at The Royal Arsenal in Woolwich, while Turpin was the Senior Draughtsman of the Design Department at Enfield. The design they came up with shared many design elements, such as its side-mounted magazine, with an earlier submachine gun design, the Lanchester (a British copy of the German MP28). Unlike the Lanchester, however, the Sten was designed to be as cheap and easy to produce as possible. To this end, the Sten was built primarily of simple stamped metal components and required only minor welding, which meant minimal machining and manufacturing was needed to build one. In terms of mechanical operation, the Sten was also incredibly simple. The Sten was a blowback-operated submachine gun firing from an open bolt, with a fixed firing pin on the face of the bolt. This means the bolt remains to the rear when the weapon is cocked, and on pulling the trigger the bolt moves forward from spring pressure, stripping the round from the magazine, chambering it, and firing the weapon all in the same movement. There is no breech locking mechanism, so the rearward movement of the bolt caused by the recoil impulse is arrested only by the mainspring and the bolt's inertia. The German {{doll name|MP40|SMG|2}}, Soviet {{doll name|PPShPPS-4143|SMG|23}}, and US {{doll name|M3|SMG|2}} submachine guns were all made with a similar design philosophy to that of the Sten: simple to make, easy to maintain, and cheap to produce.<ref name = "sten forgotten weapons">[https://www.youtube.com/watch?v=8-PmLxkOmaM Forgotten Weapons video on British submachine guns]</ref>
The Sten gun was known to suffer from a number of design faults. Stoppages were not uncommon, and could occur for a variety of reasons, some as a result of poor maintenance while others were particular to the Sten. Carbon buildup on the face of the breech or debris in the bolt raceway could cause a failure to fire, while a dirty chamber could cause a failure to feed. Firing the Sten by grasping the magazine with the supporting hand tended to wear the magazine catch, altering the angle of feed and causing a failure to feed. Additional problems stemmed from the Sten's magazine, which was a direct copy of the one used in the German MP-40. The magazine had two columns of 9mm cartridges in a staggered arrangement, merging at the top to form a single column. While other staggered magazines, such as the Thompson, fed from both the left and right side alternately, the Sten magazine required the cartridges to gradually merge at the top of the magazine to form a single column. As a consequence, any dirt or foreign matter in this taper area could cause feed malfunctions. Additionally, the walls of the magazine lip had to endure the full stresses of the rounds being pushed in by the spring. This, along with rough handling could result in deformation of the magazine lips (which required a precise 8° feed angle to operate), resulting in misfeeding and a failure to fire. The slot on the side of the body where the cocking knob ran was also a target of criticism, as the long opening could allow foreign objects to enter. The open bolt design combined with cheap manufacture and rudimentary safety devices also meant the weapon was prone to accidental discharges, which proved hazardous. A simple safety could be engaged while the bolt was in the rearwards (cocked) position. However, if a loaded Sten with the bolt in the closed position was dropped, or the butt was knocked against the ground, the bolt could move far enough rearward to pick up a round (but not far enough to be engaged by the trigger mechanism) and the spring pressure could be enough to chamber and fire the round.