We present a scanning tunneling microscope (STM) designed to operate between 275 mK and room temperature, in magnetic fields up to 14 T and in ultrahigh vacuum (UHV). The system features a compact STM connected to an UHV compatible He-3 refrigerator fitting into a bottom loading cryostat with a superconducting magnet. In this configuration, the cryostat is sitting on top of the UHV chamber, resulting in a very short distance between the STM access and the experimental position. It further enables proper thermal anchoring of the entire STM setup, allowing millikelvin temperatures to be reached in true UHV conditions. We achieve a hold time of about 40 h at 275 mK and a turnaround time of 10 h between room and base temperature. We demonstrate atomic resolution and present tunneling spectra obtained at 275 mK on the high-T-c superconductors Bi2Sr2CaCu2O8+delta and YBa2Cu3O7-delta.