A description of a wide-aperture 90-degree bending magnet is presented. This magnet was designed as a part of a beam delivery system of the heavy-ion therapy facility developed in GSI, Darmstadt, Germany. A concept of the "almost window frame" magnet with the air slots in a yoke ensures a homogeneous field (ΔB/B<±0.01%) over the area of 200×200 mm2 in the central part of the magnet in a wide range of the flux density variation. The expected integral field homogeneity of the magnet is better than ±0.02% and corresponds to the required beam positioning accuracy of ±1 mm. The dynamic behavior of the magnet is investigated. The main time constant of the magnet is calculated.