Fermilab is developing a 15 T Nb3Sn dipole demonstrator for a future very high energy pp collider as a part of the U.S. MDP Plan. The magnet design is based on optimized 60-mm aperture 4-layer shell-type coils and cold iron yoke 600 mm in diameter. The coils are graded between the inner and outer layers to maximize the performance and reduce the cost. Two mechanical structures – one, based on aluminum clamps and a thick stainless steel shell, and another one, based on a thick aluminum shell and a key&bladder assembly method – are being considered for this magnet. Magnet engineering design is complete, coil fabrication and structure procurement are in progress. Magnet design parameters, the work status and plans will be presented and discussed.