Big Bob’s Hummer is one of two vehicles owned by Big Bob Pataki, the other one being a blue 1969 Lincoln Continental; The Hummer is painted in standard military olive drab, but is likely a civilian model since military Humvees were not available for purhcase by civilians at the time. The Hummer made it's debut appearance in Arnold's Halloween, which came out in 1996; The civilian Hummer was released in 1992, and Bob mentioned that he'd owned it for a long time, so Bob's Hummer is likely a 1992-1995 model and not new.
In Hey Arnold!: The Jungle Movie, Bob is shown to still own the expensive Hummer despite having apparently lost the family home to foreclosure (due to beepers becoming obsolete and Bob's business suffering as a result); Bob likely either paid cash for the Hummer or paid off the loan when the beeper emporium was still doing good. As such, when Bob lost just about everything else, he owned the Hummer outright and was able to avoid getting it repossessed.