The F-84B, which differed from YP-84A only in having faster-firing M3 machine guns, became operational with [[14th Fighter Group]] at [[Dow Field]], [[Bangor, Maine]] in December 1947. Flight restrictions followed immediately, limiting maximum speed to [[Mach number|Mach 0.8]] due to [[control reversal]], and limiting maximum acceleration to 5.5 [[g force|''g'']] (54 m/s²) due to wrinkling of the fuselage skin. To compound the problem, parts shortages and maintenance difficulties earned the aircraft the nickname "Mechanic's Nightmare".<ref name="knaack"/> On 24 May 1948, the entire F-84B fleet was grounded due to structural failures.
[[File:14fw-f-84-1948.jpg|thumb|P-84Bs of the 48th Fighter Squadron, 14th Fighter Group, 1948.]]
A 1948 review of the entire F-84 program discovered that none of the F-84B or F-84C aircraft could be considered operational or capable of executing any aspect of their intended mission. The program was saved from cancellation because the F-84D, whose production was well underway, had satisfactorily addressed the major faults. A fly-off against the F-80 revealed that while the Shooting Star had a shorter takeoff roll, better low altitude climb rate and superior maneuverability, the F-84 could carry a greater warload, was faster, had better high altitude performance and greater range.<ref name="knaack"/> As a temporizing measure, the USAF in 1949 committed US$8 million to implement over 100 upgrades to all F-84Bs, most notably reinforcing the wings. Despite the resultant improvements, the F-84B was withdrawn from active duty by 1952.<ref name="knaack"/>