A two-dimensional numerical model of an atmospheric pressure glow (APG) discharge was developed. The simulation of the APG in helium with some nitrogen impurities successfully reproduced the discharge evolution during the breakdown process observed in experiments. The results show that the breakdown first appears at the central region of the discharge followed by the axial and radial propagation of the ionization wave. The space charge induced electric field is the main reason for the radial propagation. Penning ionization is the dominant ionization mechanism in the discharge. It leads to an elevated pre-ionization level before the discharge pulse that is essential for maintaining a uniform discharge. Increasing the driving frequency favours a transition from a filamentary to a uniform glow discharge. An increasing number of filaments with driving frequency are observed. The coalescence of the filaments at sufficiently high frequency leads to a uniform discharge. Decreasing the dielectric permittivity of the barriers limits the discharge current and leads to a shift of the discharge towards a uniform Townsend discharge.