In this talk, I will present the following result: let the viscosity $\varepsilon \rightarrow 0$ for the 2D, steady Navier-Stokes equations in the region $0 < x < L$ and $0 < y < \infty$, with no-slip boundary condition at $y = 0$. For $0 < L << 1$, we prove the validity of the steady Prandtl layer expansion for scaled Prandtl boundary layers, which include the celebrated Blasius layer. This is joint work with Yan Guo.