Uranium trioxide (UO3) is known to adopt a variety of crystalline and amorphous phases. Here we applied the Perdew-Burke-Ernzerhof functional + U formalism to predict structural, electronic, and elastic properties of five experimentally determined UO3 polymorphs, in addition to their relative stability. The simulations reveal that the methodology is well-suited to describe the different polymorphs. We found better agreement with experiment for simpler phases where all bonds are similar (alpha- and delta-), while some differences are seen for those with more complex bonding (beta-, gamma-, and eta-), which we address in terms of the disorder and defective nature of the experimental samples. Our calculations also predict the presence of uranyl bonds to affect the elastic and electronic properties. Phases containing uranyl bonds tend to have smaller band gaps and bulk moduli under 100 GPa contrary to those without uranyl bonds, which have larger band gaps and bulk moduli greater than 150 GPa. In line with experimental observations, we predict the most thermodynamically stable polymorph as gamma-UO3, the least stable as alpha-UO3, and the most stable at high pressure as gamma-UO3.