Silicon photonics is a fast growing technology for high bandwidth optical communications. Bandwidth can be strongly enhanced with the use of multimode systems, but integrated photonics still has to operate in the single-mode regime. Indeed, most multi-mode photonic devices such as waveguide bends cannot preserve the integrity of the modes. Here we present an alternative inverse-design method to design an ultra-compact multimode waveguide bend. The convergence and performance of the algorithm are studied, along with a transform method that allows to design devices made of two materials only. The optimal device is studied with full electromagnetic simulations to compare its behavior with a more traditional circular waveguide bend. A broadband transmission with very low intermodal coupling is achieved. This illustrates the potential of the objective-first inverse-design method to design ultra-compact broadband photonic devices.