High-quality graphene nanoribbons (GNRs) grown by on-surface synthesis strategies with atomic precision can be controllably doped by inserting heteroatoms or chemical groups in the molecular precursors. Here, we study the electronic structure of armchair GNRs substitutionally doped with di-boron moieties at the center, through a combination of scanning tunneling spectroscopy, angle-resolved photoemission, and density functional theory simulations. Boron atoms appear with a small displacement toward the surface, signaling their stronger interactions with the metal. We find two boron-rich flat bands emerging as impurity states inside the GNR band gap, one of them particularly broadened after its hybridization with the gold surface states. In addition, the boron atoms shift the conduction and valence bands of the pristine GNR away from the gap edge and leave unaffected the bands above and below, which become the new frontier bands and have a negligible boron character. This is due to the selective mixing of boron states with GNR bands according to their symmetry. Our results depict that the GNR band structure can be tuned by modifying the separation between di-boron moieties.