The R&D working group for the design of a B factory in the ISR tunnel has defined the characteristics of a very high luminosity machine based on a geometric separation of the beams at the interaction point (IP). Such a machine has the ISR configuration, namely two rings crossing horizontally. Each beam is made of many bunches with a relatively small number of particles power bunch and is flat at the IP. The orbits being straight in the interaction region, the synchrotron light background is maintained at a low level. In order to avoid synchro‐betatron excitation through the beam‐beam interaction and to preserve the luminosity, the bunches are rotated horizontally upstream to the IP and re‐aligned after the crossing using two deflecting rf cavities per ring (crab crossingscheme); this way, the bunches fully overlap when they collide. The commisonning of the machine would begin with symmetric energies where the beam‐beam effect is known and be progressively extended to asymmetric energies, a regime for which no experience exists yet.