Based on F90V Gundam F90 VSBR Type, the F91 is a prototype machine created under SNRI’s ‘Formula Project’. The Formula Project, also known as F Project, is a miniaturized MS development program meant to address technological problems such as the upsizing and increasing cost of MS, and to rectify Anaheim Electronics’ monopolization of the MS industry. The F91 was developed to reached the ‘performance limit tidemark of present day MS’, and as such, its capabilities exceed that of contemporary machines. SNRI only referred to it by its model number, F91. It was later given the code name "Gundam F91" by Space Ark captain Leahlee Edaberry as its face resembled the Gundam’s.
The greatest weapon employed by the Gundam F91, they are a pair of high-output large beam rifles mounted on the suit’s backpack, and powered directly by its reactor. Also known as VSBRs, they can fire either low-speed beams with high destructive power, or high-speed beams with high penetrating power, sufficient to destroy two MS with one shot or pierce a beam shield. The two VSBR typically slide under the shoulder when deployed, but can also be used to attack an enemy at the back while in storage position.