This paper presents the first direct measurement of the B meson
differential cross section, d sigma/d p_T, in ppbar collisions at sqrt s = 1.8
TeV using a sample of 19.3 +/- 0.7 pb^-1 accumulated by the Collider Detector
at Fermilab (CDF).  The cross section is measured in the central rapidity
region |y| < 1 for p_T(B) > 6.0 GeV/c by fully reconstructing the B meson
decays B^+ -> J/psi K^+ and B^0 -> J/psi K^*0(892), where J/psi -> mu^+mu^-
and K^*0 -> K^+pi^-.  A comparison is made to the theoretical QCD prediction
calculated at next-to-leading order.