From the article:
Following on from the success of the BM-13-16 Katyusha during the close of 1941, the need for a larger rocket quickly became apparent. The response to this demand was the BM-31-12. Following the standard Soviet naming convention, BM stood for ground vehicle; 31 for the rocket type / model used and the 12 for the number of rails or tubes.
The rocket was developed from an earlier design, the M30. However, the request for a longer range weapon led to the design of the M31 which had a maximum range of 4325 metres compared to the 2800 metres of the M30. This payload was delivered from a metal framework mounted on the chassis of a Studebaker US6 truck.
Each rocket was armed with a 28.9kg warhead and when combined on mass, the BM-31-12 proved extremely effective during the closing stages of the war in particular during the fierce street fighting during the Battle of Berlin.