By late 1941, Bell produced 229 P-39Fs, fitted with Aeroproducts Hydromatic propellers. One of these, or a few (sources aren't clear on this) were converted into two-seat trainers, under the designation TP-39F. All armament, both the gun and the synchronized machine guns, was removed, and the spinner was sharp-pointed. The second cockpit, fitted with only rudimentary controls for the instructor, was added in front of the original cockpit, and covered by a side-hinged canopy. The original cockpit, for the pupil, retained the same controls and instruments as the standard P-39. They communicated via an intercom telephone. The tail fillet was enlarged, and an additional ventral fin was added under the rear fuselage to increase directional stability.