Melvin Johnson was a gun designer who felt that the M1 Garand rifle had several significant flaws – so he developed his own semiauto .30-06 rifle to supplement the M1. His thought was that if problems arose with the M1 in combat, production of his rifle could provide a continuing supply of arms while problems with the M1 were worked out. The rifle he designed was a short-recoil system with a multi-lug rotating bolt (which was the direct ancestor of the AR bolt design). When the Johnson rifle was tested formally alongside the M1, the two were found to be pretty much evenly matched – which led the Army to dismiss the Johnson. If it wasn’t a significant improvement over the Garand, Ordnance didn’t see the use in siphoning off resources to produce a second rifle.
The Johnson had some interesting features – primarily its magazine design. It used a fixed 10-round rotary magazine, which could be fed by 5-round standard stripper clips or loose individual cartridges. It could also be topped up without interfering with the rifle’s action, unlike the M1. On the other hand, it was not well suited to using a bayonet, since the extra weight on the barrel was liable to cause reliability problems (since the recoil action has to be balanced for a specific reciprocating mass). Johnson thought bayonets were mostly useless, but the Army used the issue as a rationale to dismiss the Johnson from consideration.
However, Johnson was able to make sales of the rifle to the Dutch government, which was in urgent need of arms for the East Indies colonies. This is where the M1941 designation came from – it was the Dutch model name. Only a few of the 30,000 manufactured rifles were delivered before the Japanese overran the Dutch islands, rendering the rest of the shipment moot.
At this point, Johnson was also working to interest the newly-formed Marine Paratroop battalions in a light machine gun version of his rifle. The Paramarines needed an LMG which could be broken down for jumping, light enough for a single man to effectively carry, and quick to reassemble upon landing. The Johnson LMG met these requirements extremely well, and was adopted for the purpose. The Paramarines were being issues Reising folding-stock submachine guns in addition to the Johnson LMGs, and they found the Reisings less than desirable. Someone noticed that thousands of M1941 Johnson rifles (which could also have their barrel quickly and easily removed for compact storage) were effectively sitting abandoned on the docks, and the Para Marines liberated more than a few of them. These rifles were never officially on the US Army books, but they were used on Bougainville and a few other small islands.
A fellow on PracticallyShooting.com has written an excellent detailed explanation of the whole development and use of the Johnson. If that’s not enough for you, Bruce Canfield’s book is the definitive Johnson resource out there, and still readily available: