Although, I personally use B'midbar, that does not seem to be the common convention.
Consider other instances where the title is altered, such as D'varim instead of HaD'varim, *Mishpatim* instead of *HaMishpatim*, *Sh'mini* instead of *HaSh'mini*, *M'tzora'* instead of *HaM'tzora'*, and *Mattos* instead of *HamMattos*.
Also, P'kudei instead of F'kudei, and Tazria' instead of Sazria' (even according to Ashkenazim), though these are grammatical changes that sort of automatically apply when taking the words in isolation.