The MR-300 was designed as a cheap car for the masses. The idea to design this construction, one of very few automobile manufactured in Poland, arose coincidentally. At the end of 1956 the authorities decided to make use of spare production capacity at the aerospace manufacturers WSK Mielec and WSK Rzeszów. At the time, both plants were only producing planes and motorcycles. The new plan was to add automobile manufacture as well. During the initial period WSK Rzeszów prepared plans for the engine, while WSK Mielec focused on the chassis and bodywork. The project was presented at the beginning of 1957, during the National Automotive Meeting. The first prototypes were presented on July 22, 1957 in Warsaw. The new car was named Mikrus MR 300 (taking its name from the initials of the words Mielec and Rzeszów). By the end of the year, the first cars left the assembly plant. In addition, two convertible models were available.
Press reports at the time stated Taking into consideration the price, which should not exceed the price of a similar cubic capacity motorcycle by more than 25 – 30% we may assume that it will constitute a very popular means of transportation for a wide spectrum of users. The Mikrus turned out to be very popular. However, the high cost of manufacture prevented the idea from developing into a mass, individual motorization. At the same time, the high price of the vehicle was meant few could afford it. The Mikrus cost 50 thousand Polish Złoty, the average of 50 salaries. The much larger Warszawa cost 120 thousand Polish Złoty.