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The synchrotron SIS300 is a fundamental component of the FAIR facility under construction at the GSI laboratory in Darmstadt. The acceleration of high intensity proton and heavy ion beams requires 4.5 T magnets, up to 7.8 m long, to be ramped up at a rate of 1 T/s. These challenging magnets have also the particular characteristic to be geometrically curved with a sagitta of 114 mm. To demonstrate the feasibility of curved fast cycled cos-theta dipoles, R&D activities were performed at the Italian National Institute of Nuclear Physics. Important steps of the R&D have been: 1) the development of a low loss superconducting Rutherford cable; 2) the construction of coil winding models for assessing the constructive feasibility of curved coils; and 3) the construction and tests of a complete model magnet composed of a cold mass enclosed in its horizontal cryostat. During 2012 the cold mass was tested at Italian National Institute of Nuclear Physics-LASA in a vertical cryostat. The paper discusses the main issues of the R&D activity from design to manufacture, results, future developments, and future perspectives.