Are projective modules over $\mathbb{Z}[x_1,...,x_m]$ free?

Finitely generated projective modules are free over $R[x_1,\dots,x_m]$ for any PID $R$. This was proved by Quillen in his original proof; I'm not sure about Suslin's proof. See Lam's Springer monograph "Serre's Problem on Projective Modules". (In fact all projective modules are free by a 1963 result of Bass.)