Electron cooling is a well-established method to improve the phase space quality of ion beams in storage rings. In the common rest frame of the ion and the electron beam the ion is subjected to a drag force and it experiences a loss or a gain of energy which eventually reduces the energy spread of the ion beam. A calculation of this process is complicated as the electron velocity distribution is anisotropic and the cooling process takes place in a magnetic field which guides the electrons. In this paper the drag force and the energy loss are calculated in a model of binary collisions between ions and magnetized electrons, in which the Coulomb interaction is treated up to second order as a perturbation to the helical motion of the electrons. The energy loss is related to the transfer of relative velocity in a collision. Particular attention must be paid to the non-conservation of the center-of-mass motion in the presence of a magnetic field. Three kinetic regimes can be identified, depending on the relative magnitude of the distance of closest approach, the cyclotron radius and the pitch of the helical motion. Closed expressions for the energy loss are derived for monochromatic electron beams, which are folded with the velocity distribution of the electrons. Hard collisions are taken into account by regularizing the integrals with respect to the impact parameter at small distances. The resulting energy loss of the ion and the drag force are evaluated for anisotropic Maxwell velocity distributions of the electrons. The influence of the magnetic field is as follows: the energy loss is reduced if the ion moves parallel to the field, while it is enhanced if the ion velocity has a component transverse to the field. This enhancement is not as large as in the dielectric theory and in previous kinetic models.