An irreducible open
3-manifold W is R2-irreducible if it contains no non-trivial planes, i.e. given any
proper embedded plane Π in W some component of W − Π must have closure an
embedded halfspace R2× [0,∞). In this paper it is shown that if M is a connected,
P2-irreducible, open 3-manifold such that π1(M) is finitely generated and the
universal covering space M of M is R2-irreducible, then either M is homeomorphic
to R3 or π1(M) is a free product of infinite cyclic groups and fundamental
groups of closed, connected surfaces other than S2 or P2. Given any finitely
generated group G of this form, uncountably many P2-irreducible, open
3-manifolds M are constructed with π1(M)≅G such that the universal covering
space M is R2-irreducible and not homeomorphic to R3; the M are pairwise
non-homeomorphic. Relations are established between these results and the
conjecture that the universal covering space of any irredicible, orientable,
closed 3-manifold with infinite fundamental group must be homeomorphic to
R3.