The ornamental design of a bed for an aircraft, as shown and described.
FIG. 1 is an upper perspective view of the bed for an aircraft;
FIG. 2 is a front view of the bed shown in FIG. 1;
FIG. 3 is a left-end view of the bed shown in FIG. 1;
FIG. 4 is a right-end view of the bed shown in FIG. 1; and,
FIG. 5 is a top view of the bed shown in FIG. 1.
The rear view and the bottom view of the bed form no part of the claimed subject matter and are therefore omitted. The broken lines shown in the figures represent portions of the bed that form no part of the claimed design.