B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Distraction
Airspace Violation All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
On departure; about the same time we received the radio call to switch to departure; I noticed an amber SPD LMT annunciation in my PFD. Moments after the call to switch to departure; the copilot called for VNAV. Between switching frequencies and looking for any other indications of a more serious problem; I may have inadvertently pushed LNAV instead of VNAV (disengaging LNAV) which caused the flight director to fly the runway heading and not the SID. By the time the copilot engaged LNAV and turned; we apparently flew into the western edge of P56-A.Even though I briefed the warning areas; I briefed them in relation to an engine out event and to plan to go to Dulles and maybe should have been more emphatic about not flying into the prohibited airspace in general - to have the nav display in the cross scan until the turn has commenced. Double check that the right button is selected and verified. It's too easy to just fly the flight director.
[Report narrative contained no additional information.]
B737-800 flight crew reported they encroached on the P-56A restricted area when LNAV was mistakenly deselected during departure from DCA.