Modern studies on the Be star population of young open clusters point towards the presence of an evolutionary enhancement of the Be phenomenon in the second half of the B stars main sequence lifetime. However, in the galactic field, Be stars are equally present in luminosity classes V to III, indicating that there is no evolutionary trend. To investigate these diverging results we have studied samples of main sequence B stars in the galactic field and in theh and ¬ Per clusters. From the analysis of the HR diagrams it is shown that there is no segregation between class V and class III stars, both groups being evenly distributed along the whole main sequence. We conclude that luminosity classes of main sequence B stars are not related to the evolutionary status of the stars. We propose a scenario of evolutionary enhancement of the rotational velocity to explain both the lack of relation between the evolutionary status and the luminosity classes and the evolutionary enhancement of the Be phenomenon.