FLC OF A BEECH BE90; MAY HAVE PENETRATED CLASS B AIRSPACE DURING A VFR CLBOUT FROM AN ARPT UNDERNEATH THE FLOOR OF THE CLASS B AIRSPACE RESULTING IN A CONFLICT WITH A SAAB 340 DSNDING.

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AFTER TKOF FROM DFW; STARTED TURN TO EASTERLY HDG AND CONTACTED DFW APCH; WAS GIVEN SQUAWK CODE AND TOLD TO REMAIN CLR OF CLASS B AIRSPACE. CONTINUED TURN TO N; CLBING TO 3500 FT (FLOOR OF CLASS B AIRSPACE 4000 FT MSL). AFTER REACHING 3500 FT; LEVELED OFF; AND IN A FEW MOMENTS WAS GIVEN A TA. LOOKED FOR THE TFC AND SAW A SAAB 340 DSNDING THROUGH OUR ALT; I MAINTAINED SEPARATION AND CONTINUED ON. ATC THEN ASKED MY ALT AND I REPLIED 3500 FT. ATC THEN SAID I WAS IN CLASS B AIRSPACE AND TO DSND TO 2500 FT. I STARTED A DSCNT TO 2500 FT; BUT WAS SURE THE CLASS B AIRSPACE FLOOR WAS 4000 FT MSL IN THE AREA I WAS FLYING. AFTER BEING HANDED OFF TO ANOTHER CTLR; THE NEW VOICE GAVE US A PHONE NUMBER TO CALL AFTER LNDG AT OUR DEST; MCKINNEY; TX. I MADE THE CALL AND WAS TOLD THAT THE TCASII WENT OFF IN THE SAAB CAUSING A CONFLICT ALERT. I TOLD THE ATC SUPVR THAT THE FLOOR OF THE CLASS B AIRSPACE IN THAT AREA WAS 4000 FT AND I DIDN'T THINK I WAS IN VIOLATION AND HE SAID THAT ALL TCASII ALERTS REQUIRE A PHONE CALL. I THINK THE SAAB DSNDED INTO UNCTLED AIRSPACE ON THE WAY INBOUND TO DFW AND CAUSED THE TCASII TO GO OFF. I VERY RARELY FLY IN THIS AIRSPACE; BUT IN THE FUTURE I WILL FILE AN IFR FLT PLAN SO I WILL BE ALLOWED IN CLASS B AIRSPACE AND GIVEN SPACING FROM OTHER ACFT.