The use of glass in facades and roofs poses a significant challenge for engineers because glass is a linear elastic and isotropic, but also relentlessly brittle, material. To overcome this obstacle and ensure an adequate degree of safety, the principles of concrete design may be used because concrete possesses analogous mechanical characteristics, especially with respect to its brittle failure. This has led to several research and building projects involving a variety of reinforced glass beams. This idea has encouraged research on mechanically precompressed glass beams with a special focus on Spannglass Beams—glass beams with post-tensioned reinforcement. The research presented here contributes to remedying the lack of knowledge by presenting the results of four-point bending tests on 15 two-meter-long specimens with a variety of tendon diameters and initial cable forces. The experiments showed that an increase in fracture load was possible when the glass beams were precompressed. However, the cable load needed to be limited to create safe and reliable Spannglass Beams. This limitation introduces a smaller amount of elastic energy into the system, which was proven to be problematic. Additionally, the removal of sacrificial layers from the glass web turned out to be essential. Finally, the paper draws a set of conclusions by comparing precompressed glass beams with general research on glass beams and concrete design. This will allow for novel architectural opportunities that are underpinned by profound engineering: literally invisible structures will be feasible in the future, combining the astonishing mechanical properties and transparent nature of glass.