Complexity of a subset sum variant Given integers $a_1, \ldots, a_n, b \in \mathbb{N}$. What is the complexity of the following problem
$$
\exists x_1, \ldots, x_n \in \mathbb{N} \text{ such that } a_1x_1 + \ldots a_nx_n = b?
$$
I can't find this subset-sum variant in the literature.
Here are some references, not a self-contained answer to the question.  For self-contained answers, see other people’s answers and comments.

Assuming that symbol ℕ in the question denotes the set of nonnegative integers, your problem is called the feasibility version of the change-making problem.  Chapter 5 of Martello and Toth [MT90] states that it is NP-complete, attributing the result to Lueker [Lue75].  I have not checked the report [Lue75].

Even if symbol ℕ in the question denotes the set of positive integers, the problem is still NP-complete because the nonnegative version can be easily reduced to the positive version.

References

[Lue75] G. S. Lueker.  Two NP-complete problems in nonnegative integer programming.  Report No. 178, Computer Science Laboratory, Princeton University, 1975.

[MT90] Silvano Martello and Paolo Toth.  Knapsack Problems: Algorithms and Computer Implementations, Wiley, 1990.  http://www.or.deis.unibo.it/knapsack.html