Hydrogen as an essential part of future decarbonization of the energy industry makes it a crucial necessity to replace conventional, natural gas based concepts in gas turbine combustion. This paper presents an experimental study of a multi-tube jet flame burner. The study is carried out with natural gas and pure hydrogen fuel at gas turbine relevant conditions at atmospheric pressure. To identify key differences between hydrogen-air and natural gas–air flames on the overall robustness and flame flashback behavior, air bulk velocity (80–120 m/s), adiabatic flame temperature (1235–2089 K) and air inlet temperature (623–673 K) are varied over a wide range, covering a range of Reynolds numbers of 10,000–20,000. Depending on flame temperature, two different flame shapes are observed for natural gas–air flames. The shape of the hydrogen-air flame changes less over the range of flame temperatures tested, but is generally more compact. The process of fuel-air mixing is further investigated by concentration distribution measurements in a water tunnel setup. Therefore, planar laser-induced fluorescence is utilized for visualization. The measured concentration distributions confirm the overall good mixing quality but also give an explanation on the observed flashback behavior of the different burner designs at reacting tests. The findings of the study are composed in a flashback correlation combining the observed flashback drivers for the burner configurations investigated.